[llvm] Allow flang-new binary to be specified for omp fortran modules (PR #101438)

[Daniel Martinez via llvm-commits]

Calandracas606 wrote:

no point in introducing a new variable. just set the existing `LIBOMP_FORTRAN_MODULES_COMPILER` to `CACHE`

When cross compiling, the user can now pass `-DLIBOMP_FORTRAN_MODULES_COMPILER=/path/to/native/flang-new`

https://github.com/llvm/llvm-project/pull/101438