In my quest to get Rust working on PowerPC Darwin I found that LLVM never supported PowerPC Darwin, even though it does support Darwin, and PowerPC.
So methinks it can’t be that hard, although there is a port of LLVM 3.x that looks non-trivial.
But I’m just not sure what is involved. I found a book called Getting Started with LLVM Core Libraries but I’m not sure of the big picture - what does one have to do?
EDIT: one of the things that needs to be done is to emit a 10.5 compatible MachO format executable. But it seems MachOEmitter.cpp is aware of the difference already. So it looks hopeful that there isn’t much actual work to do.