madt: print CPU APIC ID as signed int

Instead of printing something like "MADT: Found CPU APIC ID 4294967295
ACPI ID 512: disabled" print the APIC ID as a singed int for a more
user-friendly -1.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45158