[infra] Fixes for cross-toolchains

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 import optparse
 import os
 import re
@@ skipping 89 matching lines @@
 def ProcessOptions(options, args):
   if options.arch == 'all':
     options.arch = 'ia32,x64,simarm,simarm64,simmips,simdbc64'
   if options.mode == 'all':
     options.mode = 'debug,release,product'
   if options.os == 'all':
     options.os = 'host,android'
   options.mode = options.mode.split(',')
   options.arch = options.arch.split(',')
   options.os = options.os.split(',')
+  if not options.gyp and options.toolchain != None:
+    print "The --toolchain flag is only supported by the gyp build."
+    print "When using the GN build, set the toolchain and sysroot using gn.py."
+    return False
   for mode in options.mode:
     if not mode in ['debug', 'release', 'product']:
       print "Unknown mode %s" % mode
       return False
   for arch in options.arch:
     archs = ['ia32', 'x64', 'simarm', 'arm', 'simarmv6', 'armv6',
              'simarmv5te', 'armv5te', 'simmips', 'mips', 'simarm64', 'arm64',
              'simdbc', 'simdbc64', 'armsimdbc', 'armsimdbc64']
     if not arch in archs:
       print "Unknown arch %s" % arch
@@ skipping 525 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())