kernel/sched: Fix SMP must-wait-for-switch conditions in abort/join

As discovered by Carlo Caione, the k_thread_join code had a case where
it detected it had been called on a thread already marked _THREAD_DEAD
and exited early.  That's not sufficient.  The thread state is mutated
from the thread itself on its exit path.  It may still be running!

Just like the code in z_swap(), we need to spin waiting on the other
CPU to write the switch handle before knowing it's safe to return,
otherwise the calling context might (and did) do something like
immediately k_thread_create() a new thread in the "dead" thread's
struct while it was still running on the other core.

There was also a similar case in k_thread_abort() which had the same
issue: it needs to spin waiting on the other CPU to kill the thread
via the same mechanism.

Fixes #58116

Originally-by: Carlo Caione <ccaione@baylibre.com>
Signed-off-by: Andy Ross <andyross@google.com>