[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.

LGTM, although another name might clarify what this is being used for, as FLANG_NEW_EXE might imply the name of the just built compiler executable.
FLANG_NEW_HOST_EXE possibly?

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