[libc] Add clock_gettime for Darwin (#167160)

This patch adds support for clock_gettime for Darwin. Darwin syscall
'gettimeofday' is used to query the time from the system.

Many headers in llvm-libc-types, namely clockid_t, struct_timespec,
struct_timeval, suseconds_t, time_t_32, time_t_64, are modified to
include
header guards as Darwin has its own implementation of primitive types.

GitOrigin-RevId: 80e4a3f7a6c3cad5ca5c1ed259c6d6813834c13f