internal/socket: use correct cmsg alignment for netbsd/arm

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>

internal/socket/sys_bsdvar.go

@@ -12,7 +12,7 @@ import (
 )
 
 func probeProtocolStack() int {
-	if runtime.GOOS == "openbsd" && runtime.GOARCH == "arm" {
+	if (runtime.GOOS == "netbsd" || runtime.GOOS == "openbsd") && runtime.GOARCH == "arm" {
 		return 8
 	}
 	var p uintptr