[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
#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.

Author: lntue

libc/include/llvm-libc-macros/stdfix-macros.h

@@ -9,12 +9,10 @@
 #ifndef __LLVM_LIBC_MACROS_STDFIX_MACROS_H
 #define __LLVM_LIBC_MACROS_STDFIX_MACROS_H
 
-#ifdef __clang__
-#if (!defined(__cplusplus) || (__clang_major__ >= 18))
+#ifdef __FRACT_FBIT__
 // _Fract and _Accum types are available
 #define LIBC_COMPILER_HAS_FIXED_POINT
-#endif // __cplusplus
-#endif // __clang__
+#endif // __FRACT_FBIT__
 
 #ifdef LIBC_COMPILER_HAS_FIXED_POINT