Revert of Remove old/unused atomicops code

tools/gn/bootstrap/bootstrap.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Bootstraps gn.
 
 It is done by first building it manually in a temporary directory, then building
 it with its own BUILD.gn to the final destination.
 """
@@ skipping 125 matching lines @@
       continue
     full_path = os.path.join(GN_ROOT, name)
     static_libraries['gn']['sources'].append(
         os.path.relpath(full_path, SRC_ROOT))
 
   static_libraries['dynamic_annotations']['sources'].extend([
       'base/third_party/dynamic_annotations/dynamic_annotations.c',
   ])
   static_libraries['base']['sources'].extend([
       'base/at_exit.cc',
+      'base/atomicops_internals_x86_gcc.cc',
       'base/base_paths.cc',
       'base/base_switches.cc',
       'base/callback_internal.cc',
       'base/command_line.cc',
       'base/debug/alias.cc',
       'base/debug/stack_trace.cc',
       'base/debug/task_annotator.cc',
       'base/environment.cc',
       'base/files/file.cc',
       'base/files/file_enumerator.cc',
@@ skipping 251 matching lines @@
     cmd.append('-v')
   cmd.append('gn')
   check_call(cmd)
 
   if not options.debug:
     check_call(['strip', os.path.join(build_dir, 'gn')])
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))