Treat R19 is a C preserved register, fixing crashing on Linux ARM64.

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 135 matching lines @@
 
   # If no cross compiler is specified, only try to figure one out on Linux.
   if not HOST_OS in ['linux']:
     raise Exception('Unless --toolchain is used cross-building is only '
                     'supported on Linux.')
 
   # For ARM Linux, by default use the Linux distribution's cross-compiler.
   if arch == 'arm':
     # To use a non-hf compiler, specify on the command line with --toolchain.
     return (DEFAULT_ARM_CROSS_COMPILER_PATH + "/arm-linux-gnueabihf")
+  if arch == 'arm64':
+    return (DEFAULT_ARM_CROSS_COMPILER_PATH + "/aarch64-linux-gnu")
 
-  # TODO(zra): Find default MIPS and ARM64 Linux cross-compilers.
+  # TODO(zra): Find default MIPS Linux cross-compiler.
 
   return None
 
 
 def SetTools(arch, target_os, options):
   toolsOverride = None
 
   toolchainprefix = GetToolchainPrefix(target_os, arch, options)
 
   # Override the Android toolchain's linker to handle some complexity in the
@@ skipping 382 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())