cmd/dist: enable plugin test on darwin/amd64

Change-Id: I6071881a5f7b9638bca0bfef76d6f4f45c9202a6
Reviewed-on: https://go-review.googlesource.com/29396
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -749,7 +749,8 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		// linux-arm64 is missing because it causes the external linker
 		// to crash, see https://golang.org/issue/17138
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm":
+		case "linux-386", "linux-amd64", "linux-arm",
+			"darwin-amd64":
 			return true
 		}
 		return false