kernel: Add new k_thread_abort()/k_thread_join()

Add a newer, much smaller and simpler implementation of abort and
join.  No need to involve the idle thread.  No need for a special code
path for self-abort.  Joining a thread and waiting for an aborting one
to terminate elsewhere share an implementation.  All work in both
calls happens under a single locked path with no unexpected
synchronization points.

This fixes a bug with the current implementation where the action of
z_sched_single_abort() was nonatomic, releasing the lock internally at
a point where the thread to be aborted could self-abort and confuse
the state such that it failed to abort at all.

Note that the arm32 and native_posix architectures, which have their
own thread abort implementations, now see a much simplified
"z_thread_abort()" internal API.

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