x86: Minor cleanup of the ISA class.
x86: Minor cleanup of the ISA class.

Remove namespace indentation, get rid of some unnecessary includes and
class prototypes, and make members consistently private.

Change-Id: If8e6375bf664c125f6776de62aefe44923f73c2e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41893
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

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