Add riscv64 build workflow

[threexc]

This tweaks the Actions pipeline to include a riscv64 build variant for Linux. It also slightly modifies the README to reflect that.

With the RISE project we are helping to build out riscv64 software + infrastructure, and more specifically building Python wheels to support some LLM/AI workflows. One of those is SGLang, which depends on python-soundfile, which in turn ultimately depends on this repository, hence this PR.