Refactoring CMake CrossCompile module.

* put most of the cross-compiling support into a function llvm_create_cross_target_internal.
* when CrossCompile is included it still generates a NATIVE target.
* llvm_create_cross_target function takes a target_name which should match a toolchain.
* llvm_create_cross_target can now be used to target more than one cross-compilation target.

llvm-svn: 232067