[libc][math] Add missing isnan variants for float16, bfloat16, and float128

The current implementations of `isnan` are limited to `float`, `double`, and `long double`. We should add support for the remaining types (`float16`, `bfloat16`, and `float128`).

Additionally, the existing implementations currently rely on `__builtin_isnan`. I think refactoring them to use `fputil` would be better, especially since `issignaling` already uses `fputil`.