arch-x86: Put misc reg indexes into a name space.

Also make them match the style guide.

Change-Id: I845f141f85d4499a5acf56c2161240764906a232
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49758
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
27 files changed