[CMake] Enable loadable modules, aka plugins, with BUILD_SHARED_LIBS on cygming.

Loadable modules could be enabled without BUILD_SHARED_LIBS with tweaks in future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212907 91177308-0d34-0410-b5e6-96231b3b80d8