kernel/sched: Fix preemption logic

The should_preempt() code was catching some of the "unrunnable" cases
but not all of them, opening the possibility of failing to preempt a
just-pended thread and thus waking it up synchronously.  There are
reports of this causing spin loops over k_poll() in the network stack
work queues (see #8049).

Note that the previous _is_dummy() call is folded into (the somewhat
verbosely named) _is_thread_prevented_from_running(), and that the
order of tests has been changed/optimized to hopefully catch common
cases earlier.

Suggested-by: Michael Scott <michael@opensourcefoundries.com>
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>