Don't filter out -O0 for debug configurations

build/toolchain.gypi

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 684 matching lines @@
             ],
           },
           'VCLinkerTool': {
             'LinkIncremental': '2',
           },
         },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" or \
             OS=="qnx"', {
             'cflags!': [
-              '-O0',
               '-O3',
               '-O2',
               '-O1',
               '-Os',
             ],
             'cflags': [
               '-fdata-sections',
               '-ffunction-sections',
             ],
           }],
@@ skipping 259 matching lines @@
                 'OptimizeReferences': '2',
                 'EnableCOMDATFolding': '2',
               },
             },
           }],  # OS=="win"
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }