[llvm] CYGWIN: Fix build of some plugins/library/unittests

CYGWIN uses the same format of WIN32 for shared libraries.
As the comment says: "a shared library can't have undefined references", in this case CYGWIN must be handled like WIN32 by CMakeLists.txt, otherwise you will get several errors when linking because some symbols are undefined.
Attached patch fixes this issue and allows to complete the build process for those targets.

Differential Revision: https://reviews.llvm.org/D154794