x86, mem: Pass the multi level page table layout in as a parameter.

Don't get it from a global constant declared in an ISA header file.

Change-Id: Ie19440abdd76500a5e12e6791e6f755ad9e95af3
Reviewed-on: https://gem5-review.googlesource.com/7344
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>