Only activate SSE2 by default on ia32

build/toolchain.gypi

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 442 matching lines @@
           'WIN32',
         ],
         # 4351: VS 2005 and later are warning us that they've fixed a bug
         #       present in VS 2003 and earlier.
         'msvs_disabled_warnings': [4351],
         'msvs_configuration_attributes': {
           'OutputDirectory': '<(DEPTH)\\build\\$(ConfigurationName)',
           'IntermediateDirectory': '$(OutDir)\\obj\\$(ProjectName)',
           'CharacterSet': '1',
         },
+      }],
+      ['OS=="win" and v8_target_arch=="ia32"', {
         'msvs_settings': {
           'VCCLCompilerTool': {
             # Ensure no surprising artifacts from 80bit double math with x86.
             'AdditionalOptions': ['/arch:SSE2'],
           },
         },
       }],
       ['OS=="win" and v8_enable_prof==1', {
         'msvs_settings': {
           'VCLinkerTool': {
@@ skipping 366 matching lines @@
                 'OptimizeReferences': '2',
                 'EnableCOMDATFolding': '2',
               },
             },
           }],  # OS=="win"
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }