[Patch] Improvements for CMake cross compilation on OS X

[Chris Bieneman]

The attached patch modifies how cross compiling is handled with CMake to make it possible to cross compile LLVM and Clang using a single CMake invocation. This patch works for LLVM & Clang with no additional patches required. It does not work for compiler-rt because of how compiler-rt's CMake configuration is implemented.

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.

I do not know how many users there are using CMake to cross compile, and I would greatly appreciate it if anyone who is cross compiling LLVM using CMake could test these patches on their configuration.

Thanks,
-Chris

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