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: Ia57e3a92e6a50cb22edde8a31e1c4e70d894dc4c
Reviewed-on: https://go-review.googlesource.com/73310
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>