Add clang-tblgen to OPTIONAL_DIRS when building native tools for the

Add clang-tblgen to OPTIONAL_DIRS when building native tools for the
cross build, so that a native version of clang-tblgen is available.
Should unbreak Clang cross build.

Also disable Polly for the native tool build, since it depends on
external libraries which may not be available, and it isn't required
anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141454 91177308-0d34-0410-b5e6-96231b3b80d8