kernel/thread_abort: Swap, don't reschedule when aborting _current

The z_reschedule() call (as of the accompanying fix) will not swap
away from a thread if called with a nested irq lock held.

But for the specific case of aborting the current thread, we
absolutely need to swap regardless of how many locks the thread that
just aborted might have held.  So call z_swap() explicitly here.

This preserves the existing z_reschedule() call in other circumstances
for compatibility with existing test cases, but adds a note explaining
why it's there when the only obvious reason for it is already covered.

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