Benchmark flang output against gfortran and distro-shipped flangs. Track expected subpar results and verify parity with system flang builds. Context: - [Comparing LLVM Flang with other Fortran compilers](https://www.linaro.org/blog/comparing-llvm-flang-with-other-fortran-compilers/) - [llvm/llvm-project#121125](https://github.com/llvm/llvm-project/issues/121125) - [Flang - the new Fortran compiler](https://blog.llvm.org/posts/2025-03-11-flang-new/)
Benchmark flang output against gfortran and distro-shipped flangs. Track expected subpar results and verify parity with system flang builds.
Context: