cortex-r4: let svc mode reuse the stack of IRQ on startup

As the svc stack is the stack of threads, there is no need to allocate a
separate stack for the startup. Reuse the IRQ stack should be OK.

Tested on rm48 board.
cortex-r4: use byte to allocate the stack

Unit of "byte" is more intuitive than "long".
Merge pull request #200 from grissiom/cortex-r4

Cortex r4