[CMake] Support for cross-compilation when build runtimes

When cross-compiling, we cannot use the just built toolchain, instead
we need to use the host toolchain which we assume has a support for
targeting the selected target platform. We also need to pass the path
to the native version of llvm-config to external projects.

Differential Revision: https://reviews.llvm.org/D41678

llvm-svn: 322046