[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 #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks to PR 36757 __builtin_signbit is now generic, so this is fixed on trunk.