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

The OpenBSD armv7 port requires 64-bit alignment for cmsgs.

Change-Id: Ibb1c5dc32c9e0851d0ee23b4ebdb316d7ed79629
Reviewed-on: https://go-review.googlesource.com/c/154397Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/socket/sys_bsdvar.go

@@ -6,9 +6,15 @@
 
 package socket
 
-import "unsafe"
+import (
+	"runtime"
+	"unsafe"
+)
 
 func probeProtocolStack() int {
+	if runtime.GOOS == "openbsd" && runtime.GOARCH == "arm" {
+		return 8
+	}
 	var p uintptr
 	return int(unsafe.Sizeof(p))
 }