stdlib: Add Workload to the stdlib

This commit adds the concept of a "Workload" to the stdlib. Workloads
specify the details needed to run a particular gem5 workload on an
stdlib board. These are specified as part of gem5-resources and loaded
via the `Workload` class though can be specified locally via the
`CustomWorkload` class.

Tests are included in this commit to verify the functionality of these
Workloads.

Change-Id: I8840d281eb01ee4138f01ee499cae96bf7e0579d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62532
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed