Implement nanosleep per https://pubs.opengroup.org/onlinepubs/009695399/basedefs/time.h.html

Tested:
Limited unit test: This makes a call and checks that no error was
returned, but we currently don't have the ability to ensure that
time has elapsed as expected.

Co-authored-by: Jeff Bailey <jeffbailey@google.com>

Reviewed By: sivachandra, jeffbailey

Differential Revision: https://reviews.llvm.org/D134095