arch: arc: add nested interrupt support

* add nested interrupt support for interrupts
   + use a varibale exc_nest_count to trace nest interrupt and exception
   + regular interrupts can be nested by regular interrupts and fast
interrupts
   + fast interrupt's priority is the highest, cannot be nested
* remove the firq stack and exception stack
   + remove the coressponding kconfig option
   + all interrupts (normal and fast) and exceptions will be handled
     in the same stack (_interrupt stack)
   + the pros are, smaller memory footprint (no firq stack), simpler
     stack management, simpler codes, etc.. The cons are, possible
     10-15 instructions overhead for the case where fast irq nests
     regular irq
* add the case of ARC in test/kernel/gen_isr_table

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>