fastmodel: Make CortexA76x1's interrupts use gem5's mechanisms.

fastmodel: Make CortexA76x1's interrupts use gem5's mechanisms.

This makes it easier to wire up CPUs to the interrupt controller, and
makes things more modular.

Change-Id: I8d3ab26e4bb588b8efb198ed145d0f58b7ee04cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21049
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>