golang.org/cl/55130 added utimensat for Solaris but didn't use it in
UtimesNano (despite indicating otherwise in the commit message). Fix
this by also using utimensat for UtimesNano on Solaris.

Because all versions of Solaris suppported by Go support utimensat,
there is no need for the fallback logic and utimensat can be called
unconditionally.

This issue was pointed out by Shawn Walker-Salas.

Updates #16480

Change-Id: I114338113a6da3cfcb8bca950674bdc8f5a7a9e5
Reviewed-on: https://go-review.googlesource.com/55141
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>