net: disable multicast test on Alpha GNU/Linux

Alpha GNU/Linux does not have /proc/net/igmp[6].

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5663047

src/pkg/net/multicast_test.go

@@ -33,7 +33,7 @@ func TestListenMulticastUDP(t *testing.T) {
 	case "netbsd", "openbsd", "plan9", "windows":
 		return
 	case "linux":
-		if runtime.GOARCH == "arm" {
+		if runtime.GOARCH == "arm" || runtime.GOARCH == "alpha" {
 			return
 		}
 	}