Rollup merge of #129111 - Zalathar:python-sysroot, r=jieyouxu

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.

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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

Author: workingjubilee