runtime: use clock_gettime(CLOCK_REALTIME) for nanosecond-precision time.now on arm64...

runtime: use clock_gettime(CLOCK_REALTIME) for nanosecond-precision time.now on arm64, mips64x

Assembly copied from the clock_gettime(CLOCK_MONOTONIC)
call in runtime.nanotime in these files and then modified to use
CLOCK_REALTIME.

Also comment system call numbers in a few other files.

Fixes #11222.

Change-Id: Ie132086de7386f865908183aac2713f90fc73e0d
Reviewed-on: https://go-review.googlesource.com/32177
Reviewed-by: Cherry Zhang <cherryyz@google.com>