kernel/sched: Don't make an IPI if we don't need it

If an architecture declares support for IPI, we still want to use it
only when running in SMP mode.

(This also fixes a build failure on ARC, which declares
CONFIG_SCHED_IPI_SUPPORTED but doesn't actually implement
z_arch_sched_ipi() yet).

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