cmd/dist: enable c-archive test on darwin/arm64

Turns out all the necessary pieces have already been submitted.

Change-Id: I19c8d614cd756821ce400ca7a338029002780b18
Reviewed-on: https://go-review.googlesource.com/9076Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/cmd/dist/test.go

@@ -381,8 +381,7 @@ func (t *tester) buildmode(mode string) bool {
 			return false
 		case t.goos == "darwin":
 			switch t.goarch {
-			case "amd64", "arm":
-				// TODO(crawshaw): add darwin/arm64
+			case "amd64", "arm", "arm64":
 				return true
 			default:
 				return false