relicense shootout-fannkuch-redux.rs

Part of #14248

Main contributors are @pcwalton, @alexcrichton and me.  Only
@dguenther appear in git blame as a minor contribution, but it is
only adding the rust license, so removed by this relicensing.
Add more description to c_str::unwrap().

It's unclear what you are supposed to do with this memory.
Let's make that more clear.