[WIP][COREV][clang] Add MAC builtins to clang+llvm

Created by: flip1995

This is based on #6 and should either be merged instead of #6 or rebased after #6 is merged. Done

TODO:

• Make builtins also available, when the +xcorev feature is enabled, not only with +xcorevmac
• Add test for lowering MAC intrinsics (LLVM IR -> Obj/ASM)
• Mark intrinsics as having side effects or something. Definitely make sure, that they don't get optimized out.