devel/llvm10: fix build with llvm17
Fix openmp linker scripts for llvm17.
Use base system libelf and don't link against libffi.
Reformat multi-line OPTION_CMAKE_ON variable.
Co-authored-by: Dimitry Andric <dim@FreeBSD.org>
Co-authored-by: Ivan Rozhuk <rozhuk.im@gmail.com>