stdlib: Remove SimpleBoard as a superclass

Previously SimpleBoard inherited from AbstractBoard and X86Board and
RiscvBoard inherited from the SimpleBoard. This has been shown to be a
needless level of abstraction. As such, this commit refactors the code
to have X86Board and RiscvBoard inherit directly from AbstractBoard.
Code common to the SimpleBoard, X86Board, and RiscvBoard has been moved
to the AbstractBoard.

Change-Id: I5a2c7404efeb4f8ddcb5d8006e3c163d10b88b2c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51790
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed