arch: arm64: Add safe exception stack init function

Add safe exception stack init function which does several things:
1) setting current cpu safe exception stack pointer to its corresponding
stack top.
2) init sp_el0 with the above safe exception stack.
That makes sure the sp_el0 points to per-cpu safe_stack in the kernel
space.
3) init the current_stack_limit and corrupted_sp with 0

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