Now that all BSD flavors support Poll, move the common implementation
into syscall_bsd.go

Change-Id: Id62a6163d41ef24e33f96540b661002b095a2fa1
Reviewed-on: https://go-review.googlesource.com/74290
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>