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

The pipe2 syscall is part of NetBSD since version 6.0 and thus exists in
all officially supported versions (6.0 through 6.1 and 7.0+).

Follows CL 38426

Change-Id: I7b62b507300c3dfbcc6ae56408a7d7088ddccc77
Reviewed-on: https://go-review.googlesource.com/94035


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>