[libc] Implement the 'nanosleep' function on the GPU

The GPU has the ability to sleep for very short periods of time. We can
map this to the existing `nanosleep` utility. This patch maps the
nanosleep utility to the existing hardware instructions as best as
possible.

Depends on D159118

Reviewed By: JonChesterfield, sivachandra

Differential Revision: https://reviews.llvm.org/D159225