box64: Enable build & dynarec on more platforms
- box64 should *work* on 64-bit little-endian Linux platforms.
I'm sure there's a way to apply functions and programmatically generate a `meta.platforms` that expresses this
(and I'll gladly accept any suggestions for this), but manually adding loongarch64 & mips64el for now.
- RISC-V received a dynarec option in 0.2.4, so enable it.