addint llvm-cbe as backend to the backend registry

[schneidersoft]

I'd like to compile llvm witrh the c backend such that i can do auto TargetPtr = llvm::TargetRegistry::lookupTarget("c", Error);

currently the instructions given do not reconfigure llvm to include the backen... ./llvm-project/llvm/build/bin/llc --version shows many backends but not "c"

How do I add it?