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

[Mark de Wever via llvm-commits]

mordante wrote:

> We have CI in place that makes this easy, and maintainers can even push to this PR directly.
Good point I forgot about that, still not entirely used to GitHub.




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