Add utimensat as a wrapper around the libc function of the same name.
utimensat was added in macOS 10.13 which is the minimum supported
release since Go 1.17 dropped support for macOS 10.12.

This also allows to drop the fallback to setattrlistTimes which was
used to set timestamps with nanosecond resolution before utimensat could
be used, see #22528 and CL 74952.

Updates #22528

Change-Id: I87b6a76acf1d642ceede9254f7d9d06dddc3fd71
Reviewed-on: https://go-review.googlesource.com/c/go/+/358274


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>