syscall: skip fork test on darwin/arm64

Just like darwin/arm.

Change-Id: Iadc30b7307ae56fd4f8a681d49672bed7ca6966f
Reviewed-on: https://go-review.googlesource.com/8810
Reviewed-by: Minux Ma <minux@golang.org>

src/syscall/syscall_unix_test.go

@@ -93,7 +93,8 @@ func TestPassFD(t *testing.T) {
 		// TODO(aram): Figure out why ReadMsgUnix is returning empty message.
 		t.Skip("skipping test on solaris, see issue 7402")
 	case "darwin":
-		if runtime.GOARCH == "arm" {
+		switch runtime.GOARCH {
+		case "arm", "arm64":
 			t.Skipf("skipping test on %d/%s, no fork", runtime.GOOS, runtime.GOARCH)
 		}
 	}