ci: add support for loongarch64-unknown-linux-gnu

[heiher]

Description

Add support for loongarch64-unknown-linux-gnu target in CI.

Source

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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