GCC 12 should have proper support for IEEE-754 compliant 128-bit
floating point in libstdc++. So warning is needed when linking against
older libstdc++ versions or LLVM libc++.
Glibc starts supporting float128 in both header and libraries since
2.32.
Reviewed By: jsji
Differential Revision: https://reviews.llvm.org/D112906
Qiu Chaofan