fastmodel: Add a utility class which makes it easier to watch signals.

fastmodel: Add a utility class which makes it easier to watch signals.

These signals come from the exported virtual subsystem and could signal
interrupts, etc. The new SignalReceiver class makes it easier to watch
those signals and perform some behavior when they change without having
to bring along a lot of systemc baggage.

Change-Id: I09651de1dd0e7340a61779aaf080c695ce299fd4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21043
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>