cmake: build monolithic libLLVMPollyLib.dylib

The module LLVMPolly.so links to that. There is really no reason to build a
large number of mini-libraries here, especially as we do have dependences
between the libraries that are not properly handled and that make linking fail
on darwin.

Submitted-by: David Fang <fang@csl.cornell.edu>
llvm-svn: 202743