fastmodel: Pull out and simplify the interrupt mechanism in the GIC.

fastmodel: Pull out and simplify the interrupt mechanism in the GIC.

This change pulls out the SPI and PPI command structures and replaces
them with a custom protocol which can deliver a SPI or PPI without
having to bundle their parameters into a structure.

Change-Id: I8f15c8b3182bd6560bf5ef0345b0bc64173def85
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21042
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>