Handle X2APIC entries in the MADT for APICs with an ID < 255. At least one
BIOS has been seen to include such entries even though the relevant specs
require that X2APIC entries only be used for CPUs with an APIC ID >= 255.

This was tested on a system with "plain" local APIC entries in the MADT
to ensure no regressions, but it has not yet been tested on a system with
X2APIC entries in the MADT. Currently such systems do not boot at all,
and with this change they might now boot correctly.

Differential Revision: https://reviews.freebsd.org/D2521
Reviewed by: kib
MFC after: 2 weeks