arch: arm: introduce macro to describe the privilege stack alignment

We introduce a new define to describe the alignment for a
privilege stack buffer. This macro definition is used by the
privilege stack generation script, to determine the required
alignment of threads' privilege stacks when building with
support for user mode.

We cannot use Z_THREAD_MIN_STACK_ALIGN in this case, because
the privilege stacks do not need to respect the minimum MPU
region alignment requirement, unless, of course, this is
enforced via the MPU Stack Guard feature.

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