unix: fix cmsg alignment on aix

This was missing from CL 171941.

Change-Id: I14f16355b2fe18103d2713faf2bebf004248fc0c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/172257
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/sockcmsg_unix.go

@@ -18,6 +18,9 @@ func cmsgAlignOf(salen int) int {
 	salign := SizeofPtr
 
 	switch runtime.GOOS {
+	case "aix":
+		// There is no alignment on AIX.
+		salign = 1
 	case "darwin", "dragonfly", "solaris":
 		// NOTE: It seems like 64-bit Darwin, DragonFly BSD and
 		// Solaris kernels still require 32-bit aligned access to