[llvm] [flang-rt] Fix flang-rt unit test linking with LLVM_ENABLE_LIBCXX (PR #139569)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

#134990 is better because it applies to projects that want to link to LLVM libraries, not just those that use the LLVM_ENABLE_RUNTIMES bootstrapping builds.

Had to be reverted because it cause buildbot failures and did not even fix [flang-aarch64-libcxx](https://lab.llvm.org/buildbot/#/builders/89) (nor will this PR).


https://github.com/llvm/llvm-project/pull/139569