cmd/objdump: skip fork test on darwin/arm64

Just like darwin/arm.

Change-Id: Ibaba67980db6e05aa71568199b2dac2fcaa86fd6
Reviewed-on: https://go-review.googlesource.com/8824Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/objdump/objdump_test.go

@@ -19,8 +19,9 @@ func buildObjdump(t *testing.T) (tmp, exe string) {
 	case "android", "nacl":
 		t.Skipf("skipping on %s", runtime.GOOS)
 	case "darwin":
-		if runtime.GOARCH == "arm" {
-			t.Skipf("skipping on %s/%s", runtime.GOOS, runtime.GOARCH)
+		switch runtime.GOARCH {
+		case "arm", "arm64":
+			t.Skipf("skipping on %s/%s, cannot fork", runtime.GOOS, runtime.GOARCH)
 		}
 	}