[libc] Skip entrypoints not present in the entrypoints list.

Summary:
If a test depends on a skipped entrypoint, then the test is also
skipped. This setup will be useful as we gradually add support for
more operating systems and target architectures.

Reviewers: asteinhauser

Differential Revision: https://reviews.llvm.org/D81489

GitOrigin-RevId: fd3295fb6f981a5c030d7540b9eda67f9c723e0f