Read the CPU ID for the current CPU from the GIC. The GIC may have a
different ID space than the kernel. Because of this we need to read the
ID from the hardware. The hardware will provide this value to the CPU by
reading any of the first 8 Interrupt Processor Targets Registers.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5706