fastmodel: Create a fake "Interrupts" object for fast model CPUs.

fastmodel: Create a fake "Interrupts" object for fast model CPUs.

This object doesn't actually manage interrupts since the fast model
CPUs do that on their own, it just checkpoints interrupt related state.

Change-Id: I9d3a6354b02e4ae7bfd032c50e51a3a841b81388
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29821
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>