sim: Create a StubWorkload for System.workload to default to.

This way there will always be a workload object, even if nothing needs
to be set up. This default can also be used in low_power_sweep.py,
where the workload object was just a placeholder.

This will allow required functionality like determining endianness of a
system into the workload, rather than (for instance) in the more generic
System object. This also makes accessing less essential functionality
simpler, because we don't have to detect whether the workload is there,
and can just return default, placeholder values from the StubWorkload.

Change-Id: Idfc3e75c65318d75a3eae6a19944ae1f79a2d111
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52103
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed