libm
These are still not yet covered in CI since we always name explicit packages there, but all crates are now part of the workspace.
cargo fmt
Apply the same formatting rules to both `libm` and `compiler-builtins`.
Distribute everything from `libm/` to better locations in the repo. `libm/libm/*` has not moved yet to avoid Git seeing the move as an edit to `Cargo.toml`. Files that remain to be merged somehow are in `etc/libm`.
