Validate against actual libm implementations

[gnzlbg]

The functions taking a single f32 could be fully validated (for all input bit-patterns) against libm.

For functions taking multiple arguments and single f64s, the validation against libm could happen for some value ranges only.

[Schultzer]

Hey @alexcrichton as far as I understand then the tests cover this right?

[alexcrichton]

I think this issue is more about exhaustive testing of all f32 bit patterns whereas today we just do a best-effort randomized fuzzing sorta against musl's libm