[Bug c++/125642] [LoongArch] `__FRACT_FBIT__` predefined in C++ mode despite `_Fract` keyword being unsupported

[wszqkzqk at qq dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125642

--- Comment #2 from Zhou Qiankang <wszqkzqk at qq dot com> ---
(In reply to Drea Pinski from comment #1)
> x86 does not support _Fract either way.

Yes, this is exactly what is expected; defining `__FRACT_FBIT__` on
LoongArch—where it is clearly not supported—is incorrect.