Add support for Canadian Cross builds where the host executables are not

Add support for Canadian Cross builds where the host executables are not
runnable on the build machine.

There are a few bits that need built for the build environment (TableGen).
This patch builds those bits, and the associated libraries, for the build
environment as well as the (usual) host environment.

Thanks to Eric C. and Devang P. for pre-commit review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56975 91177308-0d34-0410-b5e6-96231b3b80d8