Enable `f16` on platforms that were missing conversion symbols

The only requirement for `f16` support, aside from LLVM not crashing and
no ABI issues, is that symbols to convert to and from `f32` are
available. Since the update to compiler-builtins in [1], we now provide
these on all platforms.

This also enables `f16` math since there are no further requirements.

Still excluded are platforms for which LLVM emits infinitely-recursing
code.

[1]: https://github.com/rust-lang/rust/pull/125016