kernel/sched: Close hole with cross-core timeslice expirations

Moving timeslice events to timeouts isn't quite enough on SMP, as it's
still possible for systems that don't broadcast their timer interrupts
to end up handling an expiration for a foreign CPU.  There, we need an
IPI, and a symmetric call to z_time_slice() (which is itempotent and
fast) in the IPI ISR.

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