Fix iOS GN build and cleanup system_wrappers

webrtc/system_wrappers/source/atomic32_non_darwin_unix.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 33 matching lines @@
   int32_t return_value = __sync_fetch_and_sub(&value_, value);
   return_value -= value;
   return return_value;
 }
 
 bool Atomic32::CompareExchange(int32_t new_value, int32_t compare_value) {
   return __sync_bool_compare_and_swap(&value_, compare_value, new_value);
 }
 
 }  // namespace webrtc