sim: Create a Workload object for SE mode.

sim: Create a Workload object for SE mode.

The workload object is still optional for the sake of compatibility,
even though it probably shouldn't be in the long term. If a simulation
is just a collection of components with nothing in particular running on
it, for instance driven by a traffic generator, should it even have a
System object in the first place?

Change-Id: I8bcda72bdfa3730248226fb62f0bba9a83243d95
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33278
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>