Add RISC-V builds

[lengau]

Related: #219

We'd like to get RISC-V builds of Python. The current Docker images are intentionally built for an old version of Debian, so a new Docker image would need to be made for building Python on riscv64. Debian 10 (stretch) is the first version that has the necessary gcc package.

I believe the necessary steps would be:

1. Create a riscv64 Dockerfile with Debian 10
2. Add the file to the image builder
3. Add a risc-v option to build-main.py
4. Add risc-v variants to the build job

I can probably do this (and test it) at some point :-)