BPF target support

This adds `bpfel-unknown-none` and `bpfeb-unknown-none`, two new no_std targets that generate little and big endian BPF. The approach taken is very similar to the cuda target, where `TargetOptions::obj_is_bitcode` is enabled and code generation is done by the linker.

I added the targets to `dist-various-2`. There are [some tests](https://github.com/alessandrod/bpf-linker/tree/main/tests/assembly) in bpf-linker and I'm planning to add more. Those are currently not ran as part of rust CI.

Always go through the expn_that_defined query.

Rollup of 7 pull requests

Successful merges:

 - #83646 (Add a map method to Bound)
 - #85501 (Fix `deny(invalid_doc_attributes)`)
 - #85503 (rustdoc: add tooltips to some buttons)
 - #85710 (Document `From` impls in path.rs)
 - #85760 (Possible errors when accessing file metadata are platform specific)
 - #85974 (td align attribute)
 - #86014 (msp430 linker does not accept -znoexecstack. Set linker_is_gnu to fal…)

Failed merges:

 - #85972 (Rustdoc html fixes)

r? `@ghost`
`@rustbot` modify labels: rollup

msp430 linker does not accept -znoexecstack. Set linker_is_gnu to fal…

…se as workaround for now.

Tested locally and works. Closes #85948.

td align attribute

This is a follow-up of #85972. I have put this one on its own because it changes the display:

![Screenshot from 2021-06-03 21-49-11](https://user-images.githubusercontent.com/3050060/120703622-d533d280-c4b5-11eb-9519-ea1131a40bee.png)

Without align:

![Screenshot from 2021-06-03 21-49-15](https://user-images.githubusercontent.com/3050060/120703623-d5cc6900-c4b5-11eb-95f9-878d3915c7fb.png)

I also opened an issue about it: raphlinus/pulldown-cmark#533. However, I'm not sure if this is the right course of action... Should we instead ignore the warning?

r? ``@jsha``

Possible errors when accessing file metadata are platform specific

In particular the `is_dir`, `is_file` and `exists` functions suggests that querying a file requires querying the directory. On Windows this is not normally true.

r? `@m-ou-se`

Document `From` impls in path.rs

rustdoc: add tooltips to some buttons

Fix `deny(invalid_doc_attributes)`

Fixes https://github.com/rust-lang/rust/issues/85497.

Add a map method to Bound

Add a map method to std::ops::range::Bound, patterned off of the method
of the same name on Option.

Have left off creating a tracking issue initially, but as soon as I get the go-ahead from a reviewer I'll make that right away 😄

Add a map method to std::ops::range::Bound, patterned off of the method
of the same name on Option

Remove special handling of `box_free` from `LocalAnalyzer`

The special casing of `box_free` predates the use of dominators in
analyzer. It is no longer necessary now that analyzer verifies that
the first assignment dominates all uses.

Revert "shrinking the deprecated method span"

Reverts rust-lang/rust#85617

Fixes: #86000

r? `@Mark-Simulacrum`

rustdoc: link consistently to stable/beta in diagnostic messages

Builds on https://github.com/rust-lang/rust/pull/84942. This makes the diagnostics consistent with the links.

Allow raw pointers in SIMD types

Closes #85915 by loosening the strictness in typechecking and adding a test to guarantee it passes.

This still might be too strict, as references currently do pass monomorphization, but my understanding is that they are not guaranteed to be "scalar" in the same way.

Remove `doc(include)`

This nightly feature is redundant now that `extended_key_value_attributes` is stable (https://github.com/rust-lang/rust/pull/83366). `@rust-lang/rustdoc` not sure if you think this needs FCP; there was already an FCP in #82539, but technically it was for deprecating, not removing the feature altogether.

This should not be merged before #83366.

cc `@petrochenkov`

Implement DepTrackingHash for `Option` through blanket impls instead of macros

This avoids having to add a new macro call for both the `Option` and the type itself.

Noticed this while working on https://github.com/rust-lang/rust/pull/84233.
r? `@Aaron1011`

Rollup of 10 pull requests

Successful merges:

 - #83653 (Remove unused code from `rustc_data_structures::sync`)
 - #84466 (rustdoc: Remove `PrimitiveType::{to_url_str, as_str}`)
 - #84880 (Make match in `register_res` easier to read)
 - #84942 (rustdoc: link to stable/beta docs consistently in documentation)
 - #85853 (Warn against boxed DST in `improper_ctypes_definitions` lint)
 - #85939 (Fix suggestion for removing &mut from &mut macro!().)
 - #85966 (wasm: Make simd types passed via indirection again)
 - #85979 (don't suggest unsized indirection in where-clauses)
 - #85983 (Update to semver 1.0.3)
 - #85988 (Note that `ninja = false` goes under `[llvm]`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Note, the `Debug` impl is required.

Note that `ninja = false` goes under `[llvm]`

Addresses https://github.com/rust-lang/rust/issues/84938#issuecomment-852448332 - `@kornelski` does this look good?

r? `@Mark-Simulacrum` cc `@joshtriplett`

Update to semver 1.0.3

Fixes https://github.com/rust-lang/cargo/pull/9508#issuecomment-854326213 by pulling in https://github.com/dtolnay/semver/pull/247.

Fixes https://github.com/rust-lang/cargo/issues/9543

don't suggest unsized indirection in where-clauses

Skip where-clauses when suggesting using indirection in combination with
`?Sized` bounds on type parameters.

Fixes #85943.

`@estebank` I think this doesn't conflict with your work in #85947; please let me know if you'd like me to cherry pick it to a new branch based on yours instead.

wasm: Make simd types passed via indirection again

This commit updates wasm target specs to use `simd_types_indirect: true`
again. Long ago this was added since wasm simd types were always
translated to `v128` under-the-hood in LLVM, meaning that it didn't
matter whether that target feature was enabled or not. Now, however,
`v128` is conditionally used in codegen depending on target features
enabled, meaning that it's possible to get linker errors about different
signatures in code that correctly uses simd types. The fix is the same
as for all other platforms, which is to pass the type indirectly.

Fix suggestion for removing &mut from &mut macro!().

Fixes #85933

Before: (Note the suggestions.)
```
error[E0308]: mismatched types
 --> src/main.rs:2:21
  |
2 |     let _: String = &mut format!("");
  |            ------   ^^^^^^^^^^^^^^^^
  |            |        |
  |            |        expected struct `String`, found `&mut String`
  |            |        help: consider removing the borrow: `mut format!("")`
  |            expected due to this

error[E0308]: mismatched types
 --> src/main.rs:3:21
  |
3 |     let _: String = &mut (format!(""));
  |            ------   ^^^^^^^^^^^^^^^^^^
  |            |        |
  |            |        expected struct `String`, found `&mut String`
  |            |        help: consider removing the borrow: `mut (format!(""))`
  |            expected due to this
```

After:
```
error[E0308]: mismatched types
 --> src/main.rs:2:21
  |
2 |     let _: String = &mut format!("");
  |            ------   ^^^^^^^^^^^^^^^^
  |            |        |
  |            |        expected struct `String`, found `&mut String`
  |            |        help: consider removing the borrow: `format!("")`
  |            expected due to this

error[E0308]: mismatched types
 --> src/main.rs:3:21
  |
3 |     let _: String = &mut (format!(""));
  |            ------   ^^^^^^^^^^^^^^^^^^
  |            |        |
  |            |        expected struct `String`, found `&mut String`
  |            |        help: consider removing the borrow: `format!("")`
  |            expected due to this
```

Warn against boxed DST in `improper_ctypes_definitions` lint

Fixes #85714

rustdoc: link to stable/beta docs consistently in documentation

This is an alternative to https://github.com/rust-lang/rust/pull/84941 which fixes the problem consistently by linking to stable/beta for *all* items, not just for primitives.

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

cc Mark-Simulacrum - I know [you were dubious about this in the past](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Rustdoc.20unconditionally.20links.20to.20nightly.20libstd.20docs/near/231223124), but I'm not quite sure why? I see this as "just a bugfix", I don't know why rustdoc should unconditionally link to nightly.
cc dtolnay who commented in https://github.com/rust-lang/rust/issues/30693:

>  I would welcome a PR to solve this permanently if anyone has ideas for how. I don't believe we need an RFC.

Fixes https://github.com/rust-lang/rust/issues/30693 (note that issue is marked as feature-accepted, although I don't see where it was discussed).

Make match in `register_res` easier to read

- Don't duplicate DefKind -> ItemType handling; that's a good way to get bugs
- Use exhaustive match
- Add comments

This found that register_res is very wrong in at least one way: if it
registers a Res for `Variant`, it should also register one for `Field`.
But I don't know whether the one for Variant should be removed or Field
added. Maybe someone has ideas?

Found while reviewing https://github.com/rust-lang/rust/pull/84176.

rustdoc: Remove `PrimitiveType::{to_url_str, as_str}`

These can easily be rewritten in terms of `as_sym`, and this avoids bugs where the two get out of sync.

I don't expect this to have a perf impact, but I'll start a perf run just in case.

Remove unused code from `rustc_data_structures::sync`

Found using https://github.com/est31/warnalyzer. Follow-up to https://github.com/rust-lang/rust/pull/83185.

r? `@Zoxc` cc `@oli-obk`

rustc: Store metadata-in-rlibs in object files

This commit updates how rustc compiler metadata is stored in rlibs.
Previously metadata was stored as a raw file that has the same format as
`--emit metadata`. After this commit, however, the metadata is encoded
into a small object file which has one section which is the contents of
the metadata.

The motivation for this commit is to fix a common case where #83730
arises. The problem is that when rustc crates a `dylib` crate type it
needs to include entire rlib files into the dylib, so it passes
`--whole-archive` (or the equivalent) to the linker. The problem with
this, though, is that the linker will attempt to read all files in the
archive. If the metadata file were left as-is (today) then the linker
would generate an error saying it can't read the file. The previous
solution was to alter the rlib just before linking, creating a new
archive in a temporary directory which has the metadata file removed.

This problem from before this commit is now removed if the metadata file
is stored in an object file that the linker can read. The only caveat we
have to take care of is to ensure that the linker never actually
includes the contents of the object file into the final output. We apply
similar tricks as the `.llvmbc` bytecode sections to do this.

This involved changing the metadata loading code a bit, namely updating
some of the LLVM C APIs used to use non-deprecated ones and fiddling
with the lifetimes a bit to get everything to work out. Otherwise though
this isn't intended to be a functional change really, only that metadata
is stored differently in archives now.

This should end up fixing #83730 because by default dylibs will no
longer have their rlib dependencies "altered" meaning that
split-debuginfo will continue to have valid paths pointing at the
original rlibs. (note that we still "alter" rlibs if LTO is enabled to
remove Rust object files and we also "alter" for the #[link(cfg)]
feature, but that's rarely used).

Closes #83730

It's confusing for these to be different, even if some of the methods
are unused.

Found using https://github.com/est31/warnalyzer.

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel