[flang] Tweak the conditions for the GCC 7/libstdc++ workaround

This adjusts the workaround from D104731.

The issue lies in libstdc++'s classes, not GCC itself, and manifests
itself in the same way if building e.g. with clang while using
libstdc++ headers from GCC 7 (e.g. if building with Clang on Ubuntu 18.04,
while using the system default C++ library).

Therefore, change the condition to look for the version of libstdc++
instead of the compiler.

Differential Revision: https://reviews.llvm.org/D104779