mem: Maintain a back door into the AbstractMem's backing store.

mem: Maintain a back door into the AbstractMem's backing store.

The backing store pointer is added to the back door when it's set,
assuming that the range isn't interleaved. If it is interleaved, then
there isn't a way to get a flat pointer to the backing store.

Depending on how the backing store is set up, it may be possible to
return a larger backdoor which applies to all interleaved memories at
the same time and to avoid problems with interleaving. I'm leaving this
as a todo.

Change-Id: I0e531c22835ec10954ab39f761b3d87666b59220
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17668
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>