arch: Promote the PC and microPC to the PCStateBase class.
arch: Promote the PC and microPC to the PCStateBase class.

Also move up the accessors for them. By putting the storage in the base
class, we can keep the accessors non-virtual and keep their overhead
low. Subclasses will be free to set those values to whatever they want
with no overhead, just as if they were natively part of that class.

Change-Id: I58e058def174e0bf591c0a9f050c23f61e9d8823
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52037
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5