Introduce a new define to take int account the xAPIC ID limit, for
systems where x2APIC is not available/reliable.

Also change some of the usages of the APIC ID to use an unsigned int
(which is the correct storage type to deal with x2APIC IDs as found in
x2APIC MADT entries).

This allows booting FreeBSD on a box with 256 CPUs and APIC IDs up to
295:

FreeBSD/SMP: Multiprocessor System Detected: 256 CPUs
FreeBSD/SMP: 1 package(s) x 64 core(s) x 4 hardware threads
Package HW ID = 0
	Core HW ID = 0
		CPU0 (BSP): APIC ID: 0
		CPU1 (AP/HT): APIC ID: 1
		CPU2 (AP/HT): APIC ID: 2
		CPU3 (AP/HT): APIC ID: 3
[...]
	Core HW ID = 73
		CPU252 (AP): APIC ID: 292
		CPU253 (AP/HT): APIC ID: 293
		CPU254 (AP/HT): APIC ID: 294
		CPU255 (AP/HT): APIC ID: 295

Submitted by:		kib (previous version)
Relnotes:		yes
MFC after:		1 month
Reviewed by:		kib
Differential revision:	https://reviews.freebsd.org/D11913