Tested with TestPoll extracted from syscall_linux_test.go.

Now that Poll is supported on all unix flavors, TestPoll can be moved
to syscall_unix_test.go in a successive CL.

Change-Id: Ibd5ff3f45198008464522c3b165f2af7865ce81e
Reviewed-on: https://go-review.googlesource.com/73881Reviewed-by: Ian Lance Taylor <iant@golang.org>