unified/arc: Fix scheduler algorithm bug

Fix the error in thread rescheduling:

Fix Fast IRQ exit routine error when it reschedules threads if
(prio >= 0) || (sched_locked == 0) || (next_thread == _current),
while the correct condition for thread rescheduling is:
(prio >= 0) && (sched_locked == 0) && (next_thread != _current),

Fix regular IRQ error when the regular IRQ exit routine rescheduled
threads when (next_thread == _current) instead of
(next_thread != current).

Increased IDLE_STACK_SIZE for ARC architecture, to hold saved
registers.

Change-Id: I1d87a968e231e13822844b7564567e6ca310cde2
Signed-off-by: Dmitriy Korovkin <dmitriy.korovkin@windriver.com>