arch: arc: use a separate stack for exception handling

* use a separate stack for exception handling, this
will gurantee the exception handling always work, not
affected by some speical cases, e.g., stack check exception
or mem. protection exception at the exception entry.

* this commit can fix the second case of #8092

* note: the thread switch is still possible in exception
handling, e.g, caused by thread_abort. But the switched out
thread cannot be recovered, as the thread context is not
setup.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>