[libc][stdfix] Use __FRACT_FBIT__ for fixed point support detection instead of...

[libc][stdfix] Use __FRACT_FBIT__ for fixed point support detection instead of compiler version. (#81820)

Now that `__FRACT_FBIT__` and related macros are added in
https://github.com/llvm/llvm-project/pull/81207, it is safer to use them
for fixed point support detection, so that codes that do not use fixed
point types will not fail to build when `-ffixed-point` is missing.