Run GN from build.py if the output directory doesn't exist

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 81 matching lines @@
   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'
 
-  if host_os == 'win':
-    gn_args['is_clang'] = False
-  else:
-    gn_args['is_clang'] = args.clang and args.os not in ['android']
+  # TODO(zra): Investigate using clang with these configurations.
+  has_clang = (host_os != 'win'
+               and args.os not in ['android']
+               and not gn_args['target_cpu'].startswith('arm')
+               and not gn_args['target_cpu'].startswith('mips'))
+  gn_args['is_clang'] = args.clang and has_clang
 
   if args.target_sysroot:
     gn_args['target_sysroot'] = args.target_sysroot
 
   if args.toolchain_prefix:
     gn_args['toolchain_prefix'] = args.toolchain_prefix
 
   goma_dir = os.environ.get('GOMA_DIR')
   goma_home_dir = os.path.join(os.getenv('HOME', ''), 'goma')
   if args.goma and goma_dir:
@@ skipping 158 matching lines @@
 
   pool = multiprocessing.Pool(args.workers)
   results = pool.map(run_command, commands, chunksize=1)
   for r in results:
     if r != 0:
       print r.strip()
       return 1
 
   endtime = time.time()
   if args.verbose:
-    print "GN Time: " + str(endtime - starttime) + " seconds"
+    print ("GN Time: %.3f seconds" % (endtime - starttime))
   return 0
 
 
 if __name__ == '__main__':
     sys.exit(main(sys.argv))