Enable automatic cross-compilation in run-make tests
Supersedes rust-lang/rust#138066.
Blocker for rust-lang/rust#141856.
Based on rust-lang/rust#138066 plus `rustdoc()` cross-compile changes.
### Summary
This PR automatically specifies `--target` to `rustc()` and `rustdoc()` to have `rustc`/`rustdoc` produce cross-compiled artifacts in run-make tests by default, unless:
- `//@ ignore-cross-compile` is used, or
- `bare_{rustc,rustdoc}` are used, or
- Explicit `.target()` is specified, which overrides the default cross-compile target.
Some tests are necessarily modified:
- Tests that have `.target(target())` have that incantation removed (since this is now automatically the default).
- Some tests have `//@ needs-target-std`, but are a necessary-but-insufficient condition, and are changed to `//@ ignore-cross-compile` instead as host-only tests.
- A few tests received `//@ ignore-musl` that fail against `x86_64-unknown-linux-musl` because of inability to find `-lunwind`. AFAICT, they don't *need* to test cross-compiled artifacts.
- Some tests are constrained to host-only for now, because the effort to make them pass on cross-compile does not seem worth the complexity, and it's not really *meaningfully* improving test coverage.
try-job: dist-various-1
The run-make test suite contains tests which are the most flexible out of all the rust-lang/rust test suites. run-make tests can basically contain arbitrary code, and are supported by the run_make_support library.
Infrastructure
A run-make test is a test recipe source file rmake.rs accompanied by its parent directory (e.g. tests/run-make/foo/rmake.rs is the foorun-make test).
The setup for the rmake.rs can be summarized as a 3-stage process:
First, we build the run_make_support library in bootstrap as a tool lib.
Then, we compile the rmake.rs "recipe" linking the support library and its dependencies in, and provide a bunch of env vars. We setup a directory structure within build/<target>/test/run-make/
<test-name>/
rmake.exe # recipe binary
rmake_out/ # sources from test sources copied over
and copy non-rmake.rs input support files over to rmake_out/. The support library is made available as an extern prelude.
Finally, we run the recipe binary and set rmake_out/ as the working directory.
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