[libc] Make time_t 64 bits long on all platforms but arm32

This patch changes the size of time_t to be an int64_t. This still
follows the POSIX standard which only requires time_t to be an integer.

Making time_t a 64-bit integer also fixes two cases in 32 bits platforms
that use SYS_clock_nanosleep_time64 and SYS_clock_gettime64, as the name
of these calls implies, they require a 64-bit time_t. For instance, in rv32,
the 32-bit version of these syscalls is not available.

We also follow glibc here, where time_t is still a 32-bit integer in
arm32.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159125