arch: arc: fix the bug that idle thread blocks other threads

* for COOP_SCHED case, i.e., PREEMPT_ENABLED is not enabled, the
  idle thread will block other threads which is not correct.

* remove the check of PREEMPT_ENABLED in the epilogue of irq and
  exception handling. Let the scheduler (should_preempt()) decide
  whether the thread should be preempted.

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