kernel/sched: Correct IPI usage

These two spots were calling z_sched_ipi() (the IPI handler run under
the ISR, which is a noop here because obviously the current thread
isn't DEAD) and not arch_sched_ipi() (which triggers an IPI on other
CPUs to inform them of scheduling state changes), presumably because
of a typo.

Apparently we don't have tests for k_wakeup() and
k_thread_priority_set() that are sensitive to latency in SMP
contexts...

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