x86, mem: Don't try to force physical addresses on the system.

x86, mem: Don't try to force physical addresses on the system.

Use the system object to allocate physical memory instead of manually
placing certain structures and then forcing the system to start other
allocations after them in physical memory.

Change-Id: Ie18c81645c3b648c64a6d7a649a0e50f7028f344
Reviewed-on: https://gem5-review.googlesource.com/7346
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>