Widen the stored io_apic_id to 8 bits.

It seems that the newer Intel chipset did that, and Linux reads 8
bits. The only detail is that all seen datasheets, even under NDA,
claim that io apic id is 4 bits.

Submitted by: jeff
Reviewed by: jhb
Tested by: flo, pho
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D23965