syscall: don't use 32-bit aligned access for cmsgAlignOf on dragonfly after ABI change

syscall: don't use 32-bit aligned access for cmsgAlignOf on dragonfly after ABI change

Use 32-bit alignment for versions before the September 2019 ABI changes
http://lists.dragonflybsd.org/pipermail/users/2019-September/358280.html

Fixes #34958

Change-Id: Iab4b23083a7c9ca7e96a737b03e75cd36d98ee24
Reviewed-on: https://go-review.googlesource.com/c/go/+/201977
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>