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

[Nikolas Klauser via llvm-commits]

https://github.com/philnik777 commented:

LGTM, but Louis is a lot more knowledgable in his area, so I'd wait for his review.

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