arm: aarch32: introduce status bit-flag for the MPU region width

Introduce an additional single-bit flag to hold
the current length of the MPU Stack Guard size
(long or default) for the case when building
with MPU_STACK_GUARD and FPU_SHARING, in Cortex-M
platforms.

In addition, the commit adds some documentation
about the existing status flags in Arm's
thread arch.mode variable.

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