[Bug libstdc++/60637] --fast-math breaks std::signbit function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60637

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think we can fix the C++98 std::signbit(_Tp) with:

      typedef typename __gnu_cxx::__promote<_Tp>::__type __type;
      return sizeof(__type) == sizeof(long double)
        ? __builtin_signbitl(__type(__f))
        : sizeof(__type) == sizeof(double)
        ? __builtin_signbit(__type(__f))
        : __builtin_signbitf(__type(__f));