Add back support for /Gv now that gyp-win64 has VS2013

test/win/compiler-flags/calling-convention.gyp

 # Copyright (c) 2014 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
  'targets': [
     {
       'target_name': 'test_cdecl',
       'type': 'loadable_module',
       'msvs_settings': {
@@ skipping 25 matching lines @@
       'msvs_settings': {
         'VCCLCompilerTool': {
           'CallingConvention': 2,
         },
       },
       'sources': [
         'calling-convention.cc',
         'calling-convention-stdcall.def',
       ],
     },
-  ]
+  ],
+  'conditions': [
+    ['MSVS_VERSION[0:4]>="2013"', {
+      'targets': [
+        {
+          'target_name': 'test_vectorcall',
+          'type': 'loadable_module',
+          'msvs_settings': {
+            'VCCLCompilerTool': {
+              'CallingConvention': 3,
+            },
+          },
+          'sources': [
+            'calling-convention.cc',
+            'calling-convention-vectorcall.def',
+          ],
+        },
+      ],
+    }],
+  ],
 }