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

[Stephan T. Lavavej via llvm-commits]

StephanTLavavej wrote:

The CI has an unrelated failure, I believe due to #73555. I'm going to go ahead and merge this since the checks were previously OK (aside from spurious failures) and I'm 98% sure that the `multiple   spaces` will work given my previous commit to avoid emitting empty strings.

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