feat: support riscv64

[ernado]

I want to add riscv64 support for protobuf.
The protobuf is part of Kubernetes build pipeline, so this change is required to add riscv64 support to Kubernetes.

I'm not quite familiar with profobuf codebase, please guide me how I can cross-compile protobuf for riscv64, I'll test it on my devboard.

Fix: #12266

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[ernado]

Still relevant

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This PR is labeled inactive because the last activity was over 90 days ago. This PR will be closed and archived after 14 additional days without activity.

[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.