- Don't print the exact command run by rustbuild unless `--verbose` is set.
  This is almost always unhelpful, since it's just cargo with a lot of
  arguments.
- Don't print "Build completed unsuccessfully" unless --verbose is set.
  You can already tell the build failed by the errors above, and the
  time isn't particularly helpful.
- Don't print the full path to bootstrap. This is useless to everyone,
  even including when working on x.py itself. You can still opt-in to
  this being shown with `--verbose`, since it will throw an exception.

Before:

```
error[E0432]: unresolved import `x`
   --> library/std/src/lib.rs:343:5
    |
343 | use x;
    |     ^ no external crate `x`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `std`

To learn more, run the command again with --verbose.
command did not execute successfully: "/home/joshua/rustc4/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "8" "--release" "--features" "panic-unwind backtrace" "--manifest-path" "/home/joshua/rustc4/library/test/Cargo.toml" "--message-format" "json-render-diagnostics"
expected success, got: exit status: 101
failed to run: /home/joshua/rustc4/build/bootstrap/debug/bootstrap check
Build completed unsuccessfully in 0:00:13
```

After:

```
error[E0432]: unresolved import `x`
   --> library/std/src/lib.rs:343:5
    |
343 | use x;
    |     ^ no external crate `x`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `std`

To learn more, run the command again with --verbose.
```

The recursion_limit attribute avoids the following error:

```
error[E0275]: overflow evaluating the requirement `std::ptr::Unique<rustc_ast::Pat>: std::marker::Send`
  |
  = help: consider adding a `#![recursion_limit="256"]` attribute to your crate (`rustfmt_nightly`)
```

 * Make html-checker run by default on rust compiler docs as well
 * Ensure html-checker is run on CI
 * Lazify tidy binary presence check

The `RustdocGUI::should_run` condition spawns `npm list` several times
which adds up to seconds of wall-time.
Evaluate the condition lazily to to keep `./x.py test tidy` and similar
short-running tasks fast.

ignore test if rust-lld not found

create ld -> rust-lld symlink at build time instead of run time

for testing in ci

copy instead of symlinking

remove linux check

test for linker, suggestions from bjorn3

fix overly restrictive lld matcher

use -Zgcc-ld flag instead of -Clinker-flavor

refactor code adding lld to gcc path

revert ci changes

suggestions from petrochenkov

rename gcc_ld to gcc-ld in dirs

The session-derive-errors test ensures the internal SessionDiagnostic
derive macro outputs the right error messages when misused.

The macro relies on the proc_macro2 crate though, which changes its span
behavior depending on whether the channel is nightly or not. This caused
test failures when bumping the channel from nightly to beta/stable.

Since SessionDiagnostic is internal-only we don't care about its
diagnostics quality outside of nightly, as the compiler itself is
developed on nightly. Thus the easiest solution is to ignore that test
on the beta and stable channels.

This also implements `// only-{channel}` and `// ignore-{channel}` in
compiletest to properly support the change.

- Pass it explicitly where appropriate
- Update stderr files and warnings; it turns that unstable-options has
  far-reaching effects on diagnostics.

It was a trivial function only used once.

CFG_PREFIX is incorrect for rustup installed rustc versions. It also
causes unnecessary recompilation when changing the install prefix.

This reverts commit ad308264.

Cargo ignores RUSTFLAGS when building proc macro crates. However,
sometimes rustc_macro needs to have conditional compilation when there
are breaking changes to the `libproc_macro` API (see for example
tell the difference between stage 0 and stage 1.

Another alternative is to unconditionally build rustc_macros with the
master libstd instead of the beta one (i.e. use `--sysroot
stage0-sysroot`), but that led to strange and maddening errors:

```
error[E0460]: found possibly newer version of crate `std` which `proc_macro2` depends on
  --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/tracing-attributes-0.1.13/src/lib.rs:90:5
   |
90 | use proc_macro2::TokenStream;
   |     ^^^^^^^^^^^
   |
   = note: perhaps that crate needs to be recompiled?
   = note: the following crate versions were found:
           crate `std`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b3602c301b71cc3d.rmeta
           crate `proc_macro2`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps/libproc_macro2-a83c1f01610c129e.rlib
```

 ## 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

Previously, changing the standard library with `download-rustc =
"if-unchanged"` would incorrectly reuse the cached compiler and standard
library from CI, which was confusing and led to incorrect test failures
or successes.

Co-authored-by: NJosh Triplett <josh@joshtriplett.org>

These requirements change as soon as the command finishes running, and
`setup` doesn't build anything, so the check doesn't make sense.

Previously, `x.py setup` would give hard errors if `ninja` and `cmake`
were not installed, even if the new profile didn't require them.

We do not install Zlib on the CI but recent builds somehow started picking it's shared version.
To avoid relying on CI binaries so let's explicitly disable it.

Recent commits to cc have helped to address #83043 and #43468

This change adds the bpfel-unknown-none and bpfeb-unknown-none targets
which can be used to generate little endian and big endian BPF