[multilib] Teach Polly's CMake to use the libdir suffix variable. This

lets 'ninja check-polly' pass for me with a lib64 build of LLVM.

I've not updated the standalone side as I don't use it and don't have an
easy way to test any changes I've made there. I mostly wanted to be able
to actually run Polly's tests when I update its use of LLVM's APIs
during my refactorings on the (very unlikely) off chance that I make
a change which compiles but does the wrong thing.

llvm-svn: 226420