kernel: smp: introduce k_smp_cpu_start

This renames z_smp_cpu_start() to k_smp_cpu_start().
This effectively promotes z_smp_cpu_start() into a public API
which allows out of tree usage. Since this is a new API,
we can afford to change it signature, where it allows
an additional initialization steps to be done before a newly
powered up CPU starts participating in scheduling threads
to run.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>