Xtensa port: Prevent preemption of locked threads.

When an IRQ is serviced, the ISR dispatcher will check for any new thread in
the ready queue and switch to it. However, if the current thread is marked as
non preemptable due to _kernel.current->base.preempt > _NON_PREEMPT_THRESHOLD
then we should not switch to another one.

Change-Id: Icdc08105cc6433da479bb95265710462a0f37c0b
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>