syscall: use correct cmsg alignment for netbsd/arm64

syscall: use correct cmsg alignment for netbsd/arm64

netbsd/arm64 requires 128-bit alignment for cmsgs.

Re-submit of CL 258437 which was dropped due to #41718.

Change-Id: I898043d79f513bebe1a5eb931e7ebd8e291a5aec
Reviewed-on: https://go-review.googlesource.com/c/go/+/258677
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>