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

Rework the cmsg alignment code to facilitate this.

Change-Id: Ie3eae1d367e2f3ac200fa8b78e97a2eb2eccae6a
Reviewed-on: https://go-review.googlesource.com/c/153619
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>