Banshee: make register preloading configurable

Make the register preloading needed for the `startup.S` file configurable as it might differ for architectures (e.g. Mempool).
This issue is mentioned in  pulp-platform/snitch#344 and concerns these [lines](https://github.com/pulp-platform/snitch/blob/4932efac8dfe317ad8fce66bd90b3afcd1d31501/sw/banshee/src/engine.rs#L590-L593)