cpu: Pull more arch specialization to the top of BaseCPU.py.

cpu: Pull more arch specialization to the top of BaseCPU.py.

This simplifies the logic of the CPU python class, and brings us ever
so slightly closer to factoring hardcoded ISA behavior out of non-ISA
specific components.

Change-Id: I7e4511dd4e6076f5c214be5af2a0e33af0142563
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19889
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>