stdlib: Implicitly determine FS/SE mode via workload

Setting whether FS or SE mode is to be used explicitly is not necessary
as we can determine this depending on what kind of workload is set.

Change-Id: Icc0ac749df231eaa9af8ba2363b466fcb405ccea
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59571
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
6 files changed