Port the `sysroot-crates-are-unstable` Python script to rmake New version of #126231, and a follow-up to #129071. One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort. --- Part of #110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove. This is *not* part of #121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here. r? ````@jieyouxu```` try-job: aarch64-gnu try-job: aarch64-apple try-job: test-various try-job: armhf-gnu try-job: x86_64-msvc try-job: i686-mingw