arch,base,sim: Move Process loader hooks into the Process class.

arch,base,sim: Move Process loader hooks into the Process class.

This code was originally in the ObjectFile class, but not all object
files will become Processes. All Processes will ultimately come from
ObjectFiles though, so it makes more sense to put that class there.

Change-Id: Ie73e4cdecbb51ce53d24cf68911a6cfc0685d771
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21468
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabeblack@google.com>