This adds the newly released flang Fortran LLVM compiler. It needs as a companion a custom clang, added as flang-clang.

Notes:

• this is currently only for amd64 (devel/openmp is required, although we may be able to drop this requirement; patches are hardcoded for amd64 for the time being)
• OpenMP in fortran does not seem to work
• no complex program tested yet
• needs a shell script to setup some libraries correctly (i.e., ${PREFIX}/flang)

poudriere on 10.3 11.0 HEAD amd64
portlint -Aac complains about one man page in flang-clang
compiles a F77 hello world
F77 hello world prints hello world