RISC-V: Properly zero out memory when allocating

Closes RV-620

What

Fixes the memory allocation to properly zero out the entire assigned mapping.

Why

Previously we would only zero out up to the length of what was requested. However, this can be less than mapped size. Malloc from Musl, however, needs those parts to be zeroed out too.

Manually Testing

make -C src/riscv all

Benchmarking

No performance impact has been measured

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.