[Feature] RISC-V asm optimization

[Vixea]

With dav1d having optimized RISC-V asm it would be great for that to be added in rav1e. Given it looks like arm has some dav1d copyrighted asm. I would think they would be OK with RISC-V too.(correct me if I'm wrong I've only looked at this at a glance)

[lu-zero]

It needs people writing the glue code and a rebase over the current dav1d asm, if you feel like trying :)