Turn off optimization for Mac Debug builds

Review URL: https://codereview.chromium.org/11414237
Patch from Adam Klein <adamk@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13102 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/common.gypi

@@ -378,6 +378,11 @@
               }],
             ],
           }],
+          ['OS=="mac"', {
+            'xcode_settings': {
+              'GCC_OPTIMIZATION_LEVEL': '0',  # -O0
+            },
+          }],
         ],
       },  # Debug
       'Release': {