Rework the previous change to honor MADT processor IDs when probing
processor objects. Instead of forcing the new-bus CPU objects to use
a unit number equal to pc_cpuid, adjust acpi_pcpu_get_id() to honor the
MADT IDs by default. As with the previous change, setting
debug.acpi.cpu_unordered to 1 in the loader will revert to the old
behavior.

Tested by: jimharris
MFC after: 1 month