fastmodel: Add an ISA class which defers to IRIS.

fastmodel: Add an ISA class which defers to IRIS.

This class is just to enable checkpointing of "ISA" state, aka the
MiscRegs.

Change-Id: I45315b8aaa09aaf6230f44665c13597400efd780
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29822
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>