[libcxx] [llvm] [libc++][test] `ADDITIONAL_COMPILE_FLAGS` should be a space-separated list (PR #73541)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

This is nice, thanks a lot for implementing this approach!

LGTM with a tiny test coverage suggestion.

https://github.com/llvm/llvm-project/pull/73541