Fetch the current time in nanoseconds, not microseconds, by using
clock_gettime rather than gettimeofday.

Updates #11222

Change-Id: I1c2c1b88f80ae82002518359436e19099061c6fb
Reviewed-on: https://go-review.googlesource.com/26790Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Minux Ma <minux@golang.org>