arch: arc: fix the bug that interrupt stack is not switched

For the old codes, if nest interrupts come out after _isr_wrapper
and before _check_nest_int_by_irq_act, then multi-bits in irq_act
will be set, this will result irq stack will not be switched in
correctly

As a fix, it's still need to use nest interrupt counter to do
interrupt stack switch as before

The difference is in the past exc_nest_count is used, but here
_kernel.nested/_kernel.cpus[cpu_id].nested is used.

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