fastmodel: Mostly collapse ARM base classes for the CortexA76 CPU.

fastmodel: Mostly collapse ARM base classes for the CortexA76 CPU.

Fast models are in practice only ARM, so it's not that helpful to have
the ARM-ness factored out. It is, however, helpful to have aspects
which control how gem5 concepts like registers are mapped to fast model
concepts like resources, especially since these mappings may vary from
fast model to fast model.

For instance, it looks like the CortexA76 does not have predicate
vector registers. Rather than make all fast models support or not
support those registers, that can be done on a model by model basis.

Change-Id: I195da4a2f4d2f8593032d0d63e9fd3d20a240d01
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23786
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>