arch: arc: overhaul the thread switch code in epilogue of irq and exception

overhaul the thread switch code in epilogue of irq and
exception handling:
* add z_arch_get_next_switch_handle to call z_get_next_switch_handle,
  let the scheduler to decide the switch thread. This will also cover
  the case of SMP.

* put lots of common codes in macros for thread switch to improve
  the maintainablity, readability.

* clean up of some lables to make codes easier to understand

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