The OpenBSD armv7 port requires 64-bit alignment for cmsgs.

Change-Id: Ibb1c5dc32c9e0851d0ee23b4ebdb316d7ed79629
Reviewed-on: https://go-review.googlesource.com/c/154397Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>