sync/atomic: skip issue 7338 test on darwin/arm64

Similar to darwin/arm. This issue is quite worrying and I hope it
can be addressed for Go 1.5.

Change-Id: Ic095281d6a2e9a38a59973f58d464471db5a2edc
Reviewed-on: https://go-review.googlesource.com/8811Reviewed-by: Minux Ma <minux@golang.org>

src/sync/atomic/atomic_test.go

@@ -1405,7 +1405,8 @@ func TestUnaligned64(t *testing.T) {
 func TestNilDeref(t *testing.T) {
 	switch runtime.GOOS {
 	case "darwin", "freebsd", "netbsd":
-		if runtime.GOARCH == "arm" {
+		switch runtime.GOARCH {
+		case "arm", "arm64":
 			t.Skipf("issue 7338: skipping test on %s/%s", runtime.GOOS, runtime.GOARCH)
 		}
 	}