apic_enumerator: only set mp_ncpus and mp_maxid at probe cpus phase

Populate the lapics arrays and call cpu_add/lapic_create in the setup
phase instead. Also store the max APIC ID found in the newly
introduced max_apic_id global variable.

This is a requirement in order to make the static arrays currently
using MAX_LAPIC_ID dynamic.

Sponsored by: Citrix Systems R&D
MFC after: 1 month
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D11911