C API header xcvmac

This patch adds the C API header file for the xcvmac Clang builtins. It also adds the CodeGen test that checks that the wrapper functions defined in the header turn into the right LLVM instrinsic when called. Finally the patch adds the cmake support for the header file. This needs to follow the analogous patch for the xcvbitmanip builtins as that one includes the basic cmake support to add all the headers.