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

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 147 matching lines @@
   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'
 
   # TODO(zra): Investigate using clang with these configurations.
   # Clang compiles tcmalloc's inline assembly for ia32 on Linux wrong, so we
   # don't use clang in that configuration. Thus, we use gcc for ia32 *unless*
-  # a clang-based sanitizer is specified.
+  # asan or tsan is specified.
   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')
                and not ((gn_args['target_os'] == 'linux')
                         and (gn_args['host_cpu'] == 'x86')
                         and not args.asan
                         and not args.msan
                         and not args.tsan))  # Use clang for sanitizer builds.
   gn_args['is_clang'] = args.clang and has_clang
 
   gn_args['is_asan'] = args.asan and gn_args['is_clang']
   gn_args['is_msan'] = args.msan and gn_args['is_clang']
@@ skipping 59 matching lines @@
       return False
   oses = [process_os_option(os_name) for os_name in args.os]
   for os_name in oses:
     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
   return True
 
@@ skipping 166 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))