Use ppoll instead of re-implementing it using Syscall6(SYS_PPOLL, ...)

Change-Id: I7b3baa00df32dc1fe7ee80755843a492a884938c
Reviewed-on: https://go-review.googlesource.com/c/144171
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>