cmd/nm: skip fork test on darwin/arm64

Just like darwin/arm.

Change-Id: Iabb6282f18548da43117ee60f7ad6e272502f09d
Reviewed-on: https://go-review.googlesource.com/8825Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/nm/nm_test.go

@@ -59,8 +59,9 @@ func TestNM(t *testing.T) {
 	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)
 		}
 	}