kernel/sched: SWAP_NONATOMIC workaround for timeslicing

Timeslicing works by removing the _current thread from the run queue
and re-adding it at the end of its priority.  On systems with a
_Swap() that can be preempted by a timer interrupt, that means it's
possible for the timeslice to try to slice out a thread that had
already pended itself!

This behavior used to be benign (or at least undetectable) as the
duplicated list operations were idempotent.  But now the dlist code is
stricter about correctness and has exposed the bug -- it will blow up
if you try to remove an already-removed list node.

Fix (on affected platforms) by stashing the _current pointer in
_pend_current_thread() that is checked and cleared in the timer
interrupt.  If we discover we're trying to interrupt a thread that's
already interrupted itself, we can safely exit z_time_slice() as a
noop.  The timeslicing bookeeping was already done for us underneath
the pend code.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>