cmd/addr2line: skip fork test on darwin/arm64

Just like darwin/arm.

Change-Id: Ia84662f58f6b1bb168cce8a9837945b1cbd175e1
Reviewed-on: https://go-review.googlesource.com/8828Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/addr2line/addr2line_test.go

@@ -96,8 +96,9 @@ func TestAddr2Line(t *testing.T) {
 	case "nacl", "android":
 		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)
 		}
 	}