Added support for some Darwin-specific flags.
Added support for some Darwin-specific flags.

travitch/whole-program-llvm,travitch/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,BenjaminSchubert/whole-program-llvm,BenjaminSchubert/whole-program-llvm,travitch/whole-program-llvm,BenjaminSchubert/whole-program-llvm,riyadparvez/whole-program-llvm,riyadparvez/whole-program-llvm,SRI-CSL/whole-program-llvm,BenjaminSchubert/whole-program-llvm,riyadparvez/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,riyadparvez/whole-program-llvm,travitch/whole-program-llvm