kernel: Don't _arch_switch() to yourself

The SMP testing missed the case where _Swap() decides to return back
into the _current.  Obviously there is no valid switch handle for the
running thread into which we can restore, and everything blows up.
(What happened is that the new scheduler code opened up a spot where
k_thread_priority_set() does a _reschedule() unconditionally and
doens't check to see whether or not it's needed like the old code).

But that isn't incorrect!  It's entirely possible that _Swap() may
find that no thread is runnable except _current (due, for example, to
another CPU racing the other thread you expected off to sleep or
something).  Don't blow up, check and return a noop.

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