Unlike routing messages, there's no need to tweak the alignment for
socket control messages on freebsd/386 emulation.

Change-Id: Iab4a2b05080868721f7e42cebd661d445f8c1030
Reviewed-on: https://go-review.googlesource.com/44391
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>