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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

This PR assumes that flang-rt is built in a bootrapping built. When building flang-rt in a standard/standalone build using an LLVM that was built with `LLVM_ENABLE_LIBCXX=ON`, one would still get the same linking error.

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