So today I learned about cfg(target_feature="..."). I’m kinda surprised that neither I nor anyone else in this thread knew about it, it already exists since July 2015, although it’s unstable. This attribute already covers pretty much everything the hypothetical cfg(target_float="...") would have covered (other than libm stuff which was more of a random side thought), and has the advantage of already existing. It also has the disadvantage of being so naive as to be basically useless (https://github.com/rust-lang/rust/issues/31662), but whatever, that can be fixed.
The cfg(float_is_broken) alternative, on the other hand, seems less and less attractive every time I look at it. @wthrowe describes how LLVM will foil attempts at strict IEEE compatibility, the flag is a very coarse thing anyway, and now the problem it was intended to address already has another solution.
To me, that settles the topic I set out to address with this thread, though this thread has also generated other good ideas, like using openlibm!