[LIBC] Implement `sched_yield()`

Implements: https://linux.die.net/man/2/sched_yield

Possibly we don't need the return value check / errno as according to
both the manpage (and current linux source) `sched_yield` cannot fail.

Reviewed By: sivachandra, michaelrj

Differential Revision: https://reviews.llvm.org/D147985