kernel/sched: Fix edge^2 case in abort/join

The previous abort-lifecycle fix missed a case: other threads can
enter k_thread_join(), see that the thread is already dead, and then
need to call z_thread_switch_spin() to wait for a context switch.  But
the new "dummification" code was (by design!) terminating the thread
such that no context would be saved to it.  So switch_handle stayed
NULL and if you hit that timing case correctly[1] you'd deadlock
waiting for a switch that would never come.

Fix is just to set switch_handle when dummifying to any non-NULL
value.

Also add an assertion to catch the obvious case that a thread is
actually dead on the exit path of k_thread_abort() to make sure the
variant path continues to set flags correctly

[1] CI was doing it fairly reliably via tests/kernel/smp_abort on
    qemu_cortex_a53 only.  Only one of my dev systems could see it,
    and then only about 15% of the time.

Signed-off-by: Andy Ross <andyross@google.com>