cmd/go: enable -buildmode=plugin on darwin/amd64

Change-Id: I8e594e059448879a9f451801064729186ac7c11b
Reviewed-on: https://go-review.googlesource.com/29395
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -413,7 +413,8 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
-				"android/amd64", "android/arm", "android/arm64", "android/386":
+				"android/amd64", "android/arm", "android/arm64", "android/386",
+				"darwin/amd64":
 			default:
 				fatalf("-buildmode=plugin not supported on %s\n", platform)
 			}