[Bug libstdc++/58625] std::signbit always converts to double

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58625

--- Comment #16 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
That C __builtin_signbit should be type-generic is bug 36757.