Add riscv64 support

[ernado]

Add support for linux/riscv64.

Ref: #12244

[github-actions]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please add a comment.

This issue is labeled inactive because the last activity was over 90 days ago.

[ernado]

This is still relevant.

[github-actions]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please add a comment.

This issue is labeled inactive because the last activity was over 90 days ago.

[zhangskz]

Unfortunately, adding support for riscv64 would also require us to update our internal release process and images accordingly to compile for riscv64, which we are unlikely to be able to staff at the moment especially considering lack of support in manylinux currently: pypa/manylinux#1426

Per kubernetes/kubernetes#123661 (comment), it appears Kubernetes itself does not officially support riscv64 either. Closing accordingly, but we can revisit this in the future if demand changes.