detect C++ ABI flag for cpp extensions from available runtime information (#18994)

detect C++ ABI flag for cpp extensions from available runtime information (#18994)

Summary:
Previously, when a user built PyTorch from source, but set the version string manually to be binary-formatted, it would've simply used CXX11_ABI=0 incorrectly.

We have this information available at runtime with `torch._C._GLIBCXX_USE_CXX11_ABI`, so this PR improves the situation by simply using that information.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18994

Differential Revision: D14839393

Pulled By: soumith

fbshipit-source-id: ca92e0810b29ffe688be82326e02a64a5649a3ad