Add msvs_configuration_platform to common.gypi to allow x64 builds on Windows

Review URL: https://chromiumcodereview.appspot.com/10573008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11865 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/common.gypi

@@ -227,6 +227,7 @@
             'StackReserveSize': '2097152',
           },
         },
+        'msvs_configuration_platform': 'x64',
       }],  # v8_target_arch=="x64"
       ['v8_use_liveobjectlist=="true"', {
         'defines': [