arch: arc: fix the bug of irq_offload

* remove irq lock/unlock which is not needed because of
  the protection of offload_sem in irq_offload
* simplify the assembly codes related irq_offload, remove
  the thread switch logic
* the old codes may do thread switch in the epilogue of
  irq_offload handling with int locked, this is not correct
  may cause irq_offload related codes crash.

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