The submodule was causing issues in rust-lang/rust, so eliminiate it
here. `build-musl` is also removed from `libm-test`'s default features
so the crate doesn't need to be built by default.
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`.