Update x/sys module to support Risc-V

[carlosedp]

Updating projects using libraries that were recently updated to support Risc-V architecture.

Go upstream work is tracked on: golang/go#27532

Risc-V software support tracker on https://github.com/carlosedp/riscv-bringup

[derek]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide.
Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.

[alexellis]

Approved