[PowerPC] Emit warning for ieeelongdouble on older GNU toolchain

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