cpu: adjusted IRQ stack sizes for Cortex CPUs

By comparing the Cortex-based CPUs, I found that the exception stack sizes were defined quite differently. As the stack is only used on startup and when in interrupt context, it does not need to hold any context etc. So 512 byte should be quite sufficient, even in worst case scenarios. (And that is the value used on the iot-lab_M3, which we tested quite a bit...)