syscall: add S_IRWXG and S_IRWXO on OpenBSD

syscall: add S_IRWXG and S_IRWXO on OpenBSD

As discussed in CL 126621, these constants are already defined on Linux,
Darwin, FreeBSD and NetBSD. In order to ensure portability of existing
code using the syscall package, provide them for OpenBSD (and
DragonflyBSD, in a separate CL) as well.

Change-Id: Ia9e07cb01f989d144a620d268daa8ec946788861
Reviewed-on: https://go-review.googlesource.com/130336
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>