make platform filters for toolchain and target separate

[tanriol]

Thanks for the project and apologies if I'm asking for something that's already possible.

Would be nice if panamax could be configured to mirror toolchain components like rustc / cargo / rust-analyzer for a small set of platforms while mirroring target components (I think that's only rust-std, but may be missing something) for all or most platforms. I'd expect most users to use only a few platforms for running their toolchain, but they may need to target a significant number of platforms in case of something like embedded development.