fix incorrect libstdc++ from GCC >= 5.1 check
fix incorrect libstdc++ from GCC >= 5.1 check

Assume that libstdc++ from GCC >= 5.1 is being used only when compiling with GCC >= 5.1.

This unbreaks GCC 4.8.5 and 4.9.3 builds after https://codereview.chromium.org/1837563002/
landed.

Review URL: https://codereview.chromium.org/1863523005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385416}

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