MSVC fixes

fix undefined reference of ldexpl/frexpl and the cbrt fallback raising an invalid flag on nan input.
I don't think its necessary.  The motivation was, AFAICT, that since `frexp` and `ldexp` were wrapped by hand the logic for using the `double` versions for `long double` also needed to be by hand.  In npy_math we just wrap these like any other now, meaning that we always call the `double` version for `long double` if we don't have an explicit `l` version, so this logic shouldn't be necessary.