Setup LLVM crosscompiler on XCode 4.3 for various platforms by afink

I need to have a working LLVM/CLANG cross compiler setup to be used in XCode 4.3. According to my research the shipped LLVM/CLANG can already produce binaries for other platforms such as Linux/ELF but… (Budget: $30-$250 USD, Jobs: Linux, Mac OS, Objective C)