add support for riscv64

[futurejones]

Add support for riscv64 as host architecture.

Checklist

• [x] I've read the Contribution Guidelines
• [x] My contributions are licensed under the Swift license.
• [x] I've followed the coding style of the rest of the project.
• [x] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [x] I've added benchmarks covering new functionality (if appropriate).
• [x] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [x] I've updated the documentation if necessary.