[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 #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
To be clear, I'm not opposed to reusing the C++11 code for the non-standard
C++98 functions (all of them, not just signbit). That seems like a reasonable
project to undertake in stage 1, if someone is interested, but that wouldn't
help fix this bug in 4.9 and 5.