Tested with TestPoll extracted from syscall_linux_test.go. Once Poll is
supported on all OSes this test can be moved to syscall_unix_test.go.

Change-Id: I56ee0124a04c64e3ea51a1653901f2f4277983e6
Reviewed-on: https://go-review.googlesource.com/73370
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>