On linux don't take address of glX/egl symbols

* The application could export a public glXSwapBuffers symbol that is its own
  function pointer, and &glXSwapBuffers looking up that symbol will get it,
  instead of our entry point. Follow the GL function scheme of still exporting a
  small stub with the proper name, but forwarding to an internal suffixed symbol
  that won't be overloaded.