[0.2] ci: add support for loongarch64-unknown-linux-musl

[heiher]

Description

1. Fix libc-tests for loongarch64-unknown-linux-musl.
2. Add support for loongarch64-unknown-linux-musl target in CI.

Sources

None.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

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[tgross35]

I'll backport #4092 once it lands, it just needs the stable-nominated label (applied).