Revert "[infra] Roll clang to match the version used by Flutter"

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 113 matching lines @@
       return False
   options.os = [ProcessOsOption(os_name) for os_name in options.os]
   for os_name in options.os:
     if not os_name in ['android', 'freebsd', 'linux', 'macos', 'win32']:
       print "Unknown os %s" % os_name
       return False
     if os_name != HOST_OS:
       if os_name != 'android':
         print "Unsupported target os %s" % os_name
         return False
-      if not HOST_OS in ['linux', 'macos']:
+      if not HOST_OS in ['linux']:
         print ("Cross-compilation to %s is not supported on host os %s."
                % (os_name, HOST_OS))
         return False
       if not arch in ['ia32', 'x64', 'arm', 'armv6', 'armv5te', 'arm64', 'mips',
                       'simdbc', 'simdbc64']:
         print ("Cross-compilation to %s is not supported for architecture %s."
                % (os_name, arch))
         return False
       # We have not yet tweaked the v8 dart build to work with the Android
       # NDK/SDK, so don't try to build it.
@@ skipping 504 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())