Remove --host and --target arguments to configure in Dockerfiles

These arguments are passed to the relevant x.py invocation in all cases
anyway. As such, there is no need to separately configure them. x.py
will ignore the configuration when they are passed on the command line
anyway.

r? @alexcrichton

Rollup of 8 pull requests

- Successful merges: #48283, #48466, #48569, #48629, #48637, #48680, #48513, #48664
- Failed merges:

make codegen-backends directory name configurable

This allows to parallel-install several versions of rust system-wide
Fixes #48263

std: Add `arch` and `simd` modules

This commit imports the `stdsimd` crate into the standard library,
creating an `arch` and `simd` module inside of both libcore and libstd.
Both of these modules are **unstable** and will continue to be so until
RFC 2335 is stabilized.

As a brief recap, the modules are organized as so:

* `arch` contains all current architectures with intrinsics, for example
  `std::arch::x86`, `std::arch::x86_64`, `std::arch::arm`, etc. These
  modules contain all of the intrinsics defined for the platform, like
  `_mm_set1_epi8`.
* In the standard library, the `arch` module also exports a
  `is_target_feature_detected` macro which performs runtime detection to
  determine whether a target feature is available at runtime.
* The `simd` module contains experimental versions of strongly-typed
  lane-aware SIMD primitives, to be fully fleshed out in a future RFC.

The main purpose of this commit is to start pulling in all these
intrinsics and such into the standard library on nightly and allow
testing and such. This'll help allow users to easily kick the tires and
see if intrinsics work as well as allow us to test out all the
infrastructure for moving the intrinsics into the standard library.

Don't produce TOCs for doc markdown files

Currently, we are producing headers for markdown files,
which is generally not what we want. As such, passing this
flag causes them to render normally.

https://doc.rust-lang.org/nightly/book/ is an example page currently where this is done incorrectly.

Backport LLVM fixes for a JumpThreading / assume intrinsic bug

This fixes the original cause of #48116 and restores the assume intrinsic that was removed as a workaround.

r? @alexcrichton

Restore the download of rust-mingw

The build might otherwise break due to mixing MinGW object files from rust-std and the local MinGW which might be newer/older than the version used to build rust-std.

Fixes #48272

r? @alexcrichton

impl Clone for ::std_unicode::char::{ToLowercase, ToUppercase}

Improve --help performance for x.py

Since compiling the bootstrap command doesn't require any submodules,
we can skip updating submodules when a --help command is passed in.
On my machine, this saves 1 minute if the submodules are already
downloaded, and 10 minutes if run on a clean repo.

This commit also adds a message before compiling/downloading anything
when a --help command is passed in, to tell the user WHY --help
takes so long to complete. It also points the user to the bootstrap
README.md for faster help.

Finally, this fixes one warning message that still referenced using
make instead of x.py, even though x.py is now the standard way of
building rust.

Closes #37305

Disable NEON on musl ARMv7

`armv7-unknown-linux-musleabihf` target diverged a bit from `armv7-unknown-linux-gnueabihf` target. This PR re-syncs them.

Fix #47765.

add readme for librustdoc

In the same vein as the other compiler-library readmes, here's one for rustdoc! It's mainly a "how does rustdoc even" blog-post-style writeup, but i wanted to have something in-repo so people could get a sense of what bits did what.

Rollup (unsupervised)

less risky PRs, to run overnight

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Fixes #47311.
r? @nrc

Revert "correct subtle bug in the type variable code"

This reverts commit ccd92c2a.

This commit is the source of a major perf regression, and was not
intended to be included in #47861. At some point I must have
accidentally re-added the commit.

Fixes #48660.

r? @nikomatsakis

Replace Rc with Lrc for shared data

This replaces `Rc`s reachable from `TyCtxt` with `Lrc`. This has no effect unless `cfg(parallel_queries)` is set. It also contains a fix for the `Decodable` impl for `Arc`.

r? @nikomatsakis

also minor doc fixes.

Closes #43941

This commit imports the `stdsimd` crate into the standard library,
creating an `arch` and `simd` module inside of both libcore and libstd.
Both of these modules are **unstable** and will continue to be so until
RFC 2335 is stabilized.

As a brief recap, the modules are organized as so:

* `arch` contains all current architectures with intrinsics, for example
  `std::arch::x86`, `std::arch::x86_64`, `std::arch::arm`, etc. These
  modules contain all of the intrinsics defined for the platform, like
  `_mm_set1_epi8`.
* In the standard library, the `arch` module also exports a
  `is_target_feature_detected` macro which performs runtime detection to
  determine whether a target feature is available at runtime.
* The `simd` module contains experimental versions of strongly-typed
  lane-aware SIMD primitives, to be fully fleshed out in a future RFC.

The main purpose of this commit is to start pulling in all these
intrinsics and such into the standard library on nightly and allow
testing and such. This'll help allow users to easily kick the tires and
see if intrinsics work as well as allow us to test out all the
infrastructure for moving the intrinsics into the standard library.

Currently, we are producing headers for markdown files,
which is generally not what we want. As such, passing this
flag causes them to render normally.

These arguments are passed to the relevant x.py invocation in all cases
anyway. As such, there is no need to separately configure them. x.py
will ignore the configuration when they are passed on the command line
anyway.

This reverts commit ccd92c2a.

This commit is the source of a major perf regression, and was not
intended to be included in #47861. At some point I must have
accidentally re-added the commit.

This allows to parallel-install several versions of rust system-wide
Fixes #48263
Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

It was an existing solution to tell the user why a --help command
takes a long time to process. However, it would only print if the
stage0 rust compiler needed to be downloaded, it came after
update_submodules (which took a long time), and it was immediately
followed by download messages and loading bars, meaning users could
easily gloss over the message.

This commit also moves the help message out of main(), and instead
puts it at the top of bootstrap(). main() is intended to be minimal,
only handling error messages.

Another rollup

None

Rebased refactorings for Chalk

The code is Niko's, I just handled the rebase.

r? @nikomatsakis

Currently rustc isn't always the best at producing deterministic builds of a
crate when the source directory of a crate is changed. This is happening due to
what appears two different sources:

* First the `-L` paths passed to rustc are hashed into the crate hash. These
  paths through Cargo are typically absolute paths that can vary if the build
  directory changes.

* Next the paths passed to `--extern` are also hashed which like `-L` can change
  if the build directory changes.

This commit fixes these two sources of nondeterminism by ensuring that avoiding
tracking the hashes of these arguments on the command line. For `-L` paths
they're either related to loading crates (whose hashes are tracked elsewhere) or
native librarise used in the linking phase (which isn't incremental). The
`--extern` paths are similar in that they're related to crate resolution which
is already tracked independently of the command line arguments.

Closes #48019