build: use proper `-arch` for target_arch on OS X

common.gypi

@@ -170,6 +170,14 @@
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
         ],
+        'conditions': [
+          ['target_arch=="ia32"', {
+            'xcode_settings': {'ARCHS': ['i386']},
+          }],
+          ['target_arch=="x64"', {
+            'xcode_settings': {'ARCHS': ['x86_64']},
+          }],
+        ],
       }],
     ],
   }