RISC-V: Unit tests which use the Jstz kernel also use Etherlink

Relates to TZX-29.

What

This MR:

• Makes the unit tests in lib_riscv which run using the Jstz kernel also run with the Etherlink kernel
• Checks in assets and regressions for Etherlink
• Updates the dependency on riscv-pvm, test assets, and regressions

Why

Ensuring Etherlink runs correctly on the RISC-V PVM is a requirement for migration.

Manually Testing

make -C src/riscv all

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune test src/lib_riscv
  • dune build src/rust_deps
• If applicable, trigger the tezt-riscv-slow-sequential test job.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.