make the USE_ISS switch run-time instead of compile-time
Created by: strichmo
cleans up ISS usage of the tracer. Previously the USE_ISS switch defined a macro and a run-time plusarg. There is a corresponding PR for core-v-verif https://github.com/openhwgroup/core-v-verif/pull/548 but it is fine (and preferred) if this PR is merged first such that core-v-verif cv32e40x/dev points to this merged PR.