Format GN files and add a presubmit check that GN is properly formatted

tools/gn.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart 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.
 
 import argparse
 import multiprocessing
 import os
 import subprocess
 import sys
@@ skipping 73 matching lines @@
   # For Fuchsia support, the default is to not compile in the root
   # certificates.
   gn_args['dart_use_fallback_root_certificates'] = True
 
   gn_args['dart_zlib_path'] = "//runtime/bin/zlib"
 
   # Use tcmalloc only when targeting Linux and when not using ASAN.
   gn_args['dart_use_tcmalloc'] = (gn_args['target_os'] == 'linux'
                                   and not args.asan)
 
-  gn_args['arm_float_abi'] = 'hard'
+  if gn_args['target_cpu'].startswith('arm'):
+    gn_args['arm_float_abi'] = 'hard'
 
   gn_args['is_debug'] = mode == 'debug'
   gn_args['is_release'] = mode == 'release'
   gn_args['is_product'] = mode == 'product'
   gn_args['dart_debug'] = mode == 'debug'
 
   # This setting is only meaningful for Flutter. Standalone builds of the VM
   # should leave this set to 'develop', which causes the build to defer to
   # 'is_debug', 'is_release' and 'is_product'.
   gn_args['dart_runtime_mode'] = 'develop'
@@ skipping 210 matching lines @@
       return 1
 
   endtime = time.time()
   if args.verbose:
     print ("GN Time: %.3f seconds" % (endtime - starttime))
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))