CMake: fix cross-compilation with external source directories

This adds support for building native artifacts when cross-compiling using the
popular side-by-side source directory layout (no symlinks, no nested
repositories).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213091 91177308-0d34-0410-b5e6-96231b3b80d8