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
70e2b4a4d1