arch: arc: implement DIRECT IRQ support

* implement DIRECT IRQ support both for normal irq and fast irq.
* add separate interrupt stack for fast irq and use CONFIG_ARC_
  _FIRQ_STACK to control it. This will bring shortest interrupt
  latency for fast irq.
* note that scheduing in DIRECT IRQ is not supported.

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