z_sched_init: don't use arch_num_cpus()

The reason for arch_num_cpus() is to be able to dynamically adapt to
the actual number of available CPUs at run time.

In the z_sched_init() case, it is not the number of active CPUs that
we need but rather the total number of potential CPUs, and that is
represented by CONFIG_MP_MAX_NUM_CPUS not arch_num_cpus().

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>