C API header xcvbitmanip and type fixes

This patch adds the C API header file for the xcvbitmanip 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.

The patch also adds a fix for the types of the operands of some bitmanip builtins that were wrongly declared as signed instead of unsigned or as not constant arguments.

Finally the patch adds the basic cmake support for the CORE-V C API header files and adds the xcvbitmanip header file to the cmake list. The other C API header patches need to follow this.