os, syscall: use pipe2 instead of pipe syscall on OpenBSD

os, syscall: use pipe2 instead of pipe syscall on OpenBSD

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>