Prevent `rmake.rs` from using unstable features, and fix 3 run-make tests that currently do
Addresses (mostly) #137532.
Follow-up to #137373.
### Summary
- Fix 3 run-make tests that currently use unstable features:
1. `tests/run-make/issue-107495-archive-permissions/rmake.rs` uses `#![feature(rustc_private)]` for `libc` on `unix`, but `run_make_support` already exports `libc`, so just use that.
2. `tests/run-make/cross-lang-lto/rmake.rs` uses `#![feature(path_file_prefix)]` for convenience, replaced with similar filename prefix logic.
3. `tests/run-make/broken-pipe-no-ice/rmake.rs` uses `#![feature(anonymous_pipe)]` for anonymous pipes. This is more complicated[^race-condition], and I decided to temporarily introduce a dependency on [`os_pipe`] before std's `anonymous_pipe` library feature is stabilized[^pipe-stab]. I left a FIXME tracked by #137532 to make the switch once `anonymous_pipe` stabilizes and reaches beta.
- Use `RUSTC_BOOTSTRAP=-1` when building `rmake.rs` to have the stage 0 rustc reject any unstable features used in `rmake.rs`.
- The requirement that `rmake.rs` may not use any unstable features is now documented in rustc-dev-guide.
- This PR does not impose `RUSTC_BOOTSTRAP=-1` when building `run-make-support`, but I suppose we could.
r? `@Kobzol`
try-job: x86_64-msvc-1
try-job: x86_64-mingw-1
[`os_pipe`]: https://github.com/oconnor663/os_pipe.rs
[^race-condition]: We can't just try to spawn `rustc` and immediate close the stderr handle because of race condition, as there's no guarantee `rustc` will not try to print to stderr before the handle gets closed.
[^pipe-stab]: In-progress stabilization PR over at https://github.com/rust-lang/rust/pull/135822.
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