unix: add illumos case

From CL 174457 which modified this file's vendored copy in the "go"
repo.

Updates golang/go#20603

Change-Id: Ic89b2e772120a08ac0fa3a56acb93a8ee96ba337
Reviewed-on: https://go-review.googlesource.com/c/sys/+/174958
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/sockcmsg_unix.go

@@ -21,10 +21,10 @@ func cmsgAlignOf(salen int) int {
 	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
-		// network subsystem.
+	case "darwin", "dragonfly", "solaris", "illumos":
+		// NOTE: It seems like 64-bit Darwin, DragonFly BSD,
+		// illumos, and Solaris kernels still require 32-bit
+		// aligned access to network subsystem.
 		if SizeofPtr == 8 {
 			salign = 4
 		}