[gn] Move build to gypfiles

gypfiles/shim_headers.gypi

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 53 matching lines @@
       ],
       'action': ['python',
                  '<(generator_path)',
                  '<@(generator_args)',
                  '--generate',
       ],
       'message': 'Generating <(_target_name) shim headers.',
     },
   ],
 }