RISC-V: Fix type expected for maximum step counts in OCaml API

Closes RV-583

What

This MR fixes the type expected for max_steps arguments in the OCaml bindings of PVM evaluations functions to match the type passed from OCaml.

Why

Currently max_steps is passed incorrectly over the OCaml bindings, resulting in the wrong value.

How

The type is changed from usize to u64.

Manually testing the MR

A test is introduced in lib_riscv which highlights the bug (fails before the second commit):

dune build @src/lib_riscv/runtest
make -C src/riscv all

Benchmarking

The change does not impact the interpreter and should not have an impact on performance.

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 build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• 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.