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.
Currently the macro always provides `CFn`, `RustFn`, `RustArgs`, etc.
Change this so that:
1. This information must be explicily requested in the invocation.
2. There is a new `FTy` field available that emits a single float type,
rather than a tuple or signature.
Additionally, add two new macros that create enums representing function
names.
Introduce `libm_test::for_each_function`. which macro takes a callback
macro and invokes it once per function signature. This provides an
easier way of registering various tests and benchmarks without
duplicating the function names and signatures each time.
