kernel/idle: Use normal idle in SMP when IPI is available

Now that we have a working IPI framework, there's no reason for the
default spin loop for the SMP idle thread.  Just use the default
platform idle and send an IPI when a new thread is readied.

Long term, this can be optimized if necessary (e.g. only send the IPI
to idling CPUs, or check priorities, etc...), but for a 2-cpu system
this is a very reasonable default.

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