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

[David Truby via llvm-commits]

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

Sorry this fell under my radar as github didn't notify me of the change at all, nor put it back in my review list...
LGTM but I guess will need rebasing as `flang-new` is now `flang` so the default for `LIBOMP_FORTRAN_MODULES_COMPILER` will have changed.

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