Enabling LLVM & Clang to be cross-compiled using CMake from a single configuration command line

The basic idea is similar to the existing cross compilation support. A directory must be configured to build host versions of tablegen tools and llvm-config. This directory can be user provided (and configured), or it can be created during the build. During a build the native build directory will be configured and built to supply the tablegen tools used during the build. A user could also explicitly provide the tablegen executables to run on the CMake command line.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217105 91177308-0d34-0410-b5e6-96231b3b80d8