toolchain_build: Update gcc revision
toolchain_build: Update gcc revision

This brings in:

b23dd79 Fix OSX undefined _environ error in liblto_plugin.so

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3887
TEST= toolchain trybots
R=dyen@chromium.org

Review URL: https://codereview.chromium.org/705863002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@14060 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client