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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

It seems doing a complete "clean build" did help on one of the bots.
That is a bit surprising to me, as I think the ones that are still failing should "always clean". I'll have to investigate that part.

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