Once we start addinf `f16` and `f128` routines, we will need to have
this cfg for almost all uses of `for_each_function`. Rather than needing
to specify this each time, always emit `#[cfg(f16_enabled)]` or
`#[cfg(f128_enabled)]` for each function that uses `f16` or `f128`,
respectively.
The ambiguous associated types error sometimes fires in cases where it
shouldn't be ambiguous ([1]), which can make things clunky when working
with chained associated types (e.g. `Op::FTy::Int::*` does not work).
Add helper types that we can use instead of the full syntax.
There aren't too many cases in-crate now but this is relevant for some
open PRs.
[1]: https://github.com/rust-lang/rust/issues/38078
