In order to disambiguate things now that libm is part of the
compiler-builtins repository, do the following:
* Mention libm in LICENSE.txt
* Clarify the default license for crates other than libm and
compiler-builtins
* Add an explicit license field to Cargo.toml for all other crates
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`.
