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

Rework the cmsg alignment code to facilitate this.

Change-Id: I52cf55a8a4cda46c6ef35b0f694862b842028b42
Reviewed-on: https://go-review.googlesource.com/c/153837
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>