toolchain_build: Update GCC revision
toolchain_build: Update GCC revision

This brings in:

12f3bba Switch arm-nacl to use EABI unwind info

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3672
TEST= toolchain trybots
R=sehr@chromium.org

Review URL: https://codereview.chromium.org/99373004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12471 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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