[CMake][AIX] Adjust plugin library extension used on AIX

As stated in the CMake manual, we are supposed to use MODULE rules to generate
plugin libraries:

"MODULE libraries are plugins that are not linked into other targets but may be
loaded dynamically at runtime using dlopen-like functionality"

Besides, LLVM's plugin infrastructure fits with the AIX treatment of .so
shared objects more than it fits with the AIX treatment of .a library archives
(which may contain shared objects).

Differential revision: https://reviews.llvm.org/D96282