RISC-V: Fix panic hook for Rust 1.81+

Close RV-269

What

Fixes the panic hook crate such that it can compile with Rust 1.81 and later.

Why

In the RISC-V space, we need to move to very-recently-stabilised Rust versions. This is a prerequisite for that.

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.
• Benchmark performance and populate the table above if needed.
• 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.