arch: arm64: Add stack guard support for MPU

To make the stack guard works well, clean and refine the MPU code. To
save the MPU regions (the number of MPU regions are limited), we choose
to remove the guard region. Comparing to add an individual region to
guard the stack, removing the guard region can at least save 2 regions
per core.

Similarly with userspace, the stack guard will leverage the dynamic
regions switching mechanism which means we need a region switch during
the context switch. Otherwise, the other option is using stack guard
region, but this is very limited since the number of MPU regions is
limited.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>