Poll is now supported for all unix systems. Also enable TestPoll for all
of them.

Change-Id: Id6250702bf0d29c49fbeeed4cca21d232b72666a
Reviewed-on: https://go-review.googlesource.com/74310
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>