[llvm] Be explicit about what libstdc++ C++11 ABI to use (PR #154447)

[via llvm-commits]

serge-sans-paille wrote:

> Thank you. :)

Actually digging on the bot errors, it looks like the default should be to use `_GLIBCXX_USE_CXX11_ABI=1`. i'll give this a try and backout if that one fails.

https://github.com/llvm/llvm-project/pull/154447