Enable libstdc++ debug mode in debug builds.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug_mode.html for
details about the debug mode.

BUG=v8:3638
LOG=y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/648293007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24676 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/standalone.gypi

@@ -136,6 +136,9 @@
     'configurations': {
       'DebugBaseCommon': {
         'cflags': [ '-g', '-O0' ],
+        'defines': [
+          '_GLIBCXX_DEBUG'
+        ],
       },
       'Optdebug': {
         'inherit_from': [ 'DebugBaseCommon', 'DebugBase2' ],