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>

configs/example/hmc_hello.py

@@ -50,6 +50,7 @@ HMC.add_options(parser)
 options = parser.parse_args()
 # create the system we are going to simulate
 system = System()
+system.workload = SEWorkload()
 # use timing mode for the interaction between master-slave ports
 system.mem_mode = 'timing'
 # set the clock fequency of the system