cmake: Use suffix for shared modules instead of the one for shared libraries

On Linux there is no difference between shared modules and shared libaries, both
are '.so' files. However, on darwin only shared modules are '.so' files. Shared
libraries have the '.dynlib' suffix.

Fix test cases on darwin by expecting a shared module suffix for Polly instead
of a shared library suffix.

This fixes PR14135

Reported by:  Jack Howarth  <howarth@bromo.med.uc.edu>

llvm-svn: 166402