Revived the Cortex-M build, which had compiled nothing since June (#653)

Two defects, and the second hid the first.

This workflow triggered on master only, for both push and pull_request,
while dev is the integration branch. So it gated no pull request that
anybody opened. That is the same defect ports_arch_check.yml carries a
comment about, where it cost eight months of ports drifting from ports_arch
unnoticed, and regression_test.yml has it too.

And it had not compiled anything since at least 2026-06-08. Every run since
then failed in six to eight seconds at "Prepare all required actions",
before checkout, because GitHub automatically fails any request that uses
actions/cache@v1. The last run of any kind was 2026-06-30. A workflow that
fails in seven seconds is normally noticed within the day; this one was not,
because of the first defect. The two together meant the project's only job
that cross-compiles a port with GCC had been reporting nothing at all.

The toolchain now follows clang_check.yml rather than third party actions:
a pinned release fetched directly from Arm, verified against the published
sha256asc, and cached with actions/cache@v4. That also moves the compiler
off 9-2019-q4, a 2019 release, onto a version matching the GCC 14 default
this project states.

The ninja install is guarded on ninja being absent rather than run
unconditionally, because scripts/install.sh already carries a long comment
about apt-get update stalling for over two hours and taking a whole
regression run with it.

fail-fast is off so that one port failing still reports the other three.

Verified before committing, with the pinned 14.3.rel1 toolchain: the
download and sha256sum -c sequence in the install step was run as written,
the archive extracts to the directory the PATH step expects, and all four
ports configure and build clean with zero warnings.

This covers four ports of the forty under ports/ that have a gnu directory.
Widening it to every Arm gnu port is separate work.

Assisted-by: Claude Opus 5 <[email protected]>

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