Non-synthesizable statements breaking yosys based toolchains

[tarik-ibrahimovic]

Latest yosys updates added some non-synthesizable features, that caused breaking the yosys-based toolchains when synthesizing. This issue was triggered by a piece of serving core code when trying to port corescore to GateMate in this repo.

Therefore I'm makinga a feature request, to add conditional synthesis to non-synthesizable parts of code, restoring the fluidity of yosys-based toolchains, as suggested originally in this issue.