arch: arm64: Add stack check relevant variables to _cpu_arch_t

Add three per-cpu variables for the convenience of quickly accessing.

The safe_exception_stack stores the top of safe exception stack pointer.
The current_stack_limit stores the current thread's priv stack limit.
The corrputed_sp stores the priv sp or irq sp for the stack overflow
case, or 0 for the normal case.

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