Basic support for Objective-C

This is a simple commit that adds support for Objective-C and Objective-C++ sourcesets (via gcc or clang). It is at a fairly early stage but can build a Hello World for iOS just fine. Right now I'm looking mostly for feedback, for example on how to handle dependencies properly (namely frameworks, which are really just libraries but it would be nice to have a simple syntax). And on how to do this all properly, since I'm fairly new to the codebase...