FFmpeg RISC-V port


Since most of the work to support the VLC media player has been completed earlier this year, I’ve started porting the FFmpeg audio/video codec library.

Base RV64 support is now merged upstream and we also have an automatic test bot running on VisionFive v1: https://fate.ffmpeg.org (look for riscv64 architecture).

There are still a lot of potential optimizations, e.g. adding const and restrict qualifiers to help the C compiler code generation in tight loops, but sadly the FFmpeg project is mainly interested in vector optimisations at this point.