runtime: use clock_gettime for time.now() on s390x

runtime: use clock_gettime for time.now() on s390x

This should improve the precision of time.now() from microseconds
to nanoseconds.

Also, modify runtime.nanotime to keep it consistent with cleanup
done to time.now.

Updates #11222 for s390x.

Change-Id: I27864115ea1fee7299360d9003cd3a8355f624d3
Reviewed-on: https://go-review.googlesource.com/27710
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>