Fix SIMMIPS build with latest XCode (Thanks Ivan!)

runtime/tools/gyp/runtime-configurations.gypi

 # 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.
 
 {
   'variables': {
     'dart_debug_optimization_level%': '2',
     # If we have not set dart_io_support to 1 in Dart's all.gypi or common.gypi,
     # then do not build the native libraries supporting  dart:io.
     'dart_io_support%': 0,
@@ skipping 81 matching lines @@
         ],
       },
 
       'Dart_simarm_Base': {
         'abstract': 1,
         'xcode_settings': {
           'ARCHS': [ 'i386' ],
         },
       },
 
+      'Dart_simmips_Base': {
+        'abstract': 1,
+        'xcode_settings': {
+          'ARCHS': [ 'i386' ],
+        },
+      },
+
       'Dart_Release': {
         'abstract': 1,
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }