Port bug 864262 - Allow to do 32-bits build in x86-64 mock environments r=bustage-fix
Port Bug 859966 - Switch linux builds to gcc 4.7.2 to fix make check bustage. r=bustage fix
Force downgrade the linux builds to gcc4.5 for now, whilst releng upgrade our machines so we can use 4.7. r=bustage-fix.