Reland: [libc][POSIX][pthreads] implemented missing pthread_rwlockattr functions (#93622)

New pull request for https://github.com/llvm/llvm-project/issues/89443

The previous PR was reverted after breaking fullbuild due to a missing
struct declaration, which I forgot to commit.

Reverts revert and adds the missing pthread_rwlockattr_getkind_np /
pthread_rwlockattr_setkind_np functions and tests respecitvely.