fastmodel: Templatize the xn versions of the CortexA76.

fastmodel: Templatize the xn versions of the CortexA76.

This will make it a lot easier and more succinct to define the x2-x4
versions of that CPU.

Change-Id: I951cd3af4419c62892c57968e729fd11c0e4a59e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21503
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>