Mac OS X 10.13 introduced APFS which stores nanosecond resolution
timestamps. The implementation of os.Stat already returns full
resolution timestamps, but os.Chtimes only sets timestamps with
microsecond resolution.

Fix this by using setattrlist on Darwin, which takes a struct timeval
with nanosecond resolution. This is what Mac OS X 10.13 appears uses
to implement utimensat, according to dtruss.

Fixes #22528

Change-Id: I397dabef6b2b73a081382999aa4c4405ab8c6015
Reviewed-on: https://go-review.googlesource.com/74952
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>