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>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202743 91177308-0d34-0410-b5e6-96231b3b80d8