linux_like: unify SIGEV_THREAD_ID support

[christoph-heiss]

This has been supported in musl since release v1.2.2 in January 2021. See also the commit in musl itself.

Not sure about emscripten though - I guess without gating it, it would be enabled there too? As this is my first contribution, please just holler if something is off.

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[JohnTitor]

(Should be merged after we upgrade musl header on CI).

[christoph-heiss]

(Should be merged after we upgrade musl header on CI).

For reference, should be mergeable after this PR: #3068.

[tgross35]

@rustbot blocked