The pipe2 syscall is part of OpenBSD since version 5.7 and thus exists in
all officially supported versions.

Follows CL 38426 and CL 94035

Change-Id: I8f93ecbc89664241f1b6b0d069e948776941b1d0
Reviewed-on: https://go-review.googlesource.com/97356
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>