x/sys/unix: allow nil argument slice for Utimes on Linux
x/sys/unix: allow nil argument slice for Utimes on Linux

Fixes golang/go#11830.

Change-Id: Ie7ffd4579a7b8143ca85a30d24acd3e2e7eece78
Reviewed-on: https://go-review.googlesource.com/12648
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

darkliquid/golang-sys,multiplay/golang-sys,zhaoming200808/sys,golang/sys,kim-racktop/sys,EricLagergren/sys-1,stevenh/golang-sys,stevenh/golang-sys,multiplay/golang-sys,golang/sys,darkliquid/golang-sys,kim-racktop/sys,multiplay/golang-sys,darkliquid/golang-sys,zhaoming200808/sys,npat-efault/sys,kim-racktop/sys,kim-racktop/sys,multiplay/golang-sys,stevenh/golang-sys,golang/sys,darkliquid/golang-sys,EricLagergren/sys-1,zhaoming200808/sys,npat-efault/sys,EricLagergren/sys-1,npat-efault/sys,stevenh/golang-sys,npat-efault/sys,EricLagergren/sys-1,zhaoming200808/sys