arch-x86,cpu: Don't use aliases to hide TheISA::.

arch-x86,cpu: Don't use aliases to hide TheISA::.

We need to gradually eliminate TheISA, and so it's helpful to know where
it's actually being used. This change stops hiding it behind using-s
and, in one case, a placeholder constant.

Change-Id: I391a3129256a9f7bd3b4002d0a46fb06b3068468
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39656
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>