fastmodel: Refactor the CortexA76x1 model for MP support.

fastmodel: Refactor the CortexA76x1 model for MP support.

This change inverts the relationship between the fast model and gem5
CPUs, and factors out the parts of the CortexA76x1 which are per core
vs. per cluster.

Change-Id: I33eacc2461f08c7fd1784936b230e96c768c0e79
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21501
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>