arc: the caculation of exception stack is wrong

[vonhust]

Describe the bug
The caculation of exception stack in arch/arc/core/fault_s.S's _exc_entry is wrong.
'''
add sp, sp, EXCEPTION_STACK_SIZE
'''
will cause the stack overflow.

This will make exception handling corrupted.

Why does this issue comes out ? Because no run of full sanitycheck test.

That's a lesson to learn!!!