Merge rust-lang/libm#84

84: Implement fmod r=japaric a=P1n3appl3

closes rust-lang/libm#21 
I replaced the `isnanf()` function from `fmodf()` with a call to the core function `is_nan()`. If there's a reason we needed to re-implement it then I can change that back.

Also I couldn't figure out what to do in `src/lib.rs`. Is the `#[cfg(todo]` that I should be removing the one for `mod_euc()`? If so why is the equivalent one for the `f64` version still there?

Co-authored-by: Joseph Ryan <josephryan3.14@gmail.com>
Co-authored-by: Jorge Aparicio <jorge@japaric.io>

library/compiler-builtins/libm/test-generator/src/main.rs

@@ -725,7 +725,7 @@ f64_f64! {
 f64f64_f64! {
     // atan2,
     fdim,
-    // fmod,
+    fmod,
     hypot,
     // pow,
 }