Fix gn build

We need to update the call to make_link.py in the same way the GYP build was
updated.

pkg/BUILD.gn

@@ -18,15 +18,17 @@ action("pkg") {
   timestamp_file = "$target_gen_dir/packages.stamp"
   outputs = [
     timestamp_file,
+    "$root_out_dir/.packages",
   ]
 
   script = "../tools/make_links.py"
   args = [
+    "--create-links",
+    "--create-package-file",
     "--quiet",
     "--timestamp_file",
     rebase_path(timestamp_file),
-    rebase_path("$root_out_dir/packages"),] +
-    inputs +
+  ] + inputs +
     # Pub imports dart2js as compiler_unsupported so it can work outside
     # the SDK. Map that to the compiler package.
     [rebase_path("compiler/lib") + ":compiler_unsupported",]