These are never available in musl, so introduce easier ways to skip them rather than needing to exclude f16/f128 functions in three different places.
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`.
