scripts: gen_priv_stacks.py: use the right alignment for priv stacks

The privilege stacks are not sandboxed inside an MPU region,
so they do not have to be aligned with the stack buffer size.
We fix this by using the PRIVILEGE_STACK_ALIGN macro, which
is defined in arch.h and reflects the minimum alignment
requirement for privilege stack buffers.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>