ninja&make/mac: Only pass -x for loadable_modules.

test/mac/strip/main.c

+// Copyright (c) 2013 Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+static void the_static_function() {}
+__attribute__((used)) void the_used_function() {}
+
+__attribute__((visibility("hidden"))) __attribute__((used))
+void the_hidden_function() {}
+__attribute__((visibility("default"))) __attribute__((used))
+void the_visible_function() {}
+
+void the_function() {}
+
+extern const int eci;
+__attribute__((used)) int i;
+__attribute__((used)) const int ci = 34623;
+
+int main() {
+  the_function();
+  the_static_function();
+  the_used_function();
+  the_hidden_function();
+  the_visible_function();
+}