Support RISC-V

Running in free hardware is a necessary step for a local-first decentralized economy.

Let's support the RISC-V architecture in this project, following the mev-boost steps.

Start by reviewing how the project currently targets supported architectures and the referenced mev-boost steps; the issue names no files, tests, or entry points. Define the RISC-V support requirements and use the mev-boost work as the comparison point. Done means the project supports RISC-V as specified and its relevant checks pass.

Written by the indexing model from the issue text.