GN: Fix cross ARM64 and cross MIPS builds.

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 122 matching lines @@
     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
                                   and not args.msan
                                   and not args.tsan)
 
-  # Force -mfloat-abi=hard and -mfpu=neon on Linux as we're specifying
-  # a gnueabihf compiler in //build/toolchain/linux BUILD.gn.
-  # TODO(zra): This will likely need some adjustment to build for armv6 etc.
-  hard_float = (gn_args['target_cpu'].startswith('arm') and
-                (gn_args['target_os'] == 'linux'))
-  if hard_float:
-    gn_args['arm_float_abi'] = 'hard'
-    gn_args['arm_use_neon'] = True
+  if gn_args['target_os'] == 'linux':
+    if gn_args['target_cpu'] == 'arm':
+      # Force -mfloat-abi=hard and -mfpu=neon for arm on Linux as we're
+      # specifying a gnueabihf compiler in //build/toolchain/linux BUILD.gn.
+      gn_args['arm_arch'] = 'armv7'
+      gn_args['arm_float_abi'] = 'hard'
+      gn_args['arm_use_neon'] = True
+    elif gn_args['target_cpu'] == 'armv6':
+      raise Exception("GN support for armv6 unimplemented")
+    elif gn_args['target_cpu'] == 'armv5te':
+      raise Exception("GN support for armv5te unimplemented")
 
   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 255 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))