RISC-V: system call constants for secp256k1 verify and keccak256 hash

Closes RV-691

What

Introduces two constants which identify the secp256k1 verification and keccak256 hash system calls. These have already been implemented in the riscv pvm. The relevant (merged) github PR is here https://github.com/tezos/riscv-pvm/pull/93

Why

For host crypto for etherlink, and more immediately the revm kernel

How

add to constants to kernel sdk

Manually Testing

(irrelevant)

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.