arch: arc: fix the bug in exception return for secure mode

* current codes use a faked interrupt return to do a thread switch
  in exception return.
* so the different between exception return and interrupt return
  should be carefully considered.
* when secure is enabled, the sec_stat should also be pushed in
  exception entry.
* when secure is enabled, SEC_STAT.IRM must be configured corrrectly
  for interrupt return.
* For the case of faked interrupt return in exception return, the
  correct value of SEC_STAT.IRM comes from ER_SEC_STAT.ERM.

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