The NetBSD arm port requires 64-bit alignment for cmsgs.

Change-Id: I619a205b4ff1b292b4d9eb2565799fe2c0a41b9f
Reviewed-on: https://go-review.googlesource.com/c/164777
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>