[libcxx] [llvm] [libc++] Fix checks for terminal and flushes in std::print() (PR #70321)

[Nikolas Klauser via llvm-commits]

https://github.com/philnik777 approved this pull request.

Thanks! Do you have commit access, or should I merge the patch?

https://github.com/llvm/llvm-project/pull/70321