Field stack_info.size is supposed to hold the actual writable
thread stack area, above what is pointed to by stack_info.start.
Therefore, in all MPU architectures we can program the Thread
Stack MPU region as starting from thread->stack_obj and with
size equal to stack_info.size plus the (possible) difference
between stack_obj and stack_info.start.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>