Simplify CI & expand coverage

[datdenkikniet]

Instead of having a bunch of duplicate tasks, we can simply use/abuse the github matrices a little.

There's a bunch of repetition for platform/backend/toolchain combinations, but at least it's obvious what is and isn't being tested. We should be able to replace them with a YAML anchor once that's supported

Also add coverage for RISCV platforms/backends in clippy and check.

[korken89]

Nice improvement! The CI files were getting unwieldy. The only thing I think we should do in the future is to move as much as possible of all this into xtask or similar so one can do this locally and not need to have CI do it every time.

[datdenkikniet]

Yeah, this is initial clean up. I plan to move the book scripts into actual script files, and have asked @AfoHT to look over the QEMU setup (since I'm not sure the prebuilt binaries are really being used).

ETA: ah, and yes, moving this matrix into xtask would be good.