[libc] Unify gettime implementations (#65383)

Similar to D159208, this patch unifies the calls to a syscall, in this
patch it is the syscall SYS_clock_gettime/SYS_clock_gettime64.

This patch also fixes calls to SYS_clock_gettime64 by creating a
timespec64 object, passing it to the syscall and rewriting the timespec
given by the caller with timespec64 object's contents. This fixes cases
where timespec has a 4 bytes long time_t member, but SYS_clock_gettime
is not available (e.g., rv32).