[libc] Implement gettimeofday

Implement gettimeofday per
.../onlinepubs/9699919799/functions/gettimeofday.html.
This call clock_gettime to implement gettimeofday function.

Tested:
Limited unit test: This makes a call and checks that no error was
returned. Used nanosleep for 100 microseconds and verfified it
returns a value that elapses more than 100 microseconds and less
than 300 microseconds.

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

Differential Revision: https://reviews.llvm.org/D137881