unix: fix TestFchmodat on illumos
unix: fix TestFchmodat on illumos

Like linux/android, illumos doesn't support Fchmodat flags != 0.

The comment already mentions this, so update the condition as well now
that GOOS illumos is known.

Change-Id: Ic6542223129ec5fd8bb56dab3c8a042dc29ec7cc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/179362
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys