[AIX] Add %pluginext and update tests to use proper pluginext

As a follow on to D96282, since bug point passes is built as a module the proper file extension to use is LLVM_PLUGIN_EXT, rather than SHLIBEXT. Using SHLIBEXT causes the tests to load a non-existent file on AIX. We also adjust the PluginsTest unittest  to use LLVM_PLUGIN_EXT for similar reasons.

This change should hopefully make little difference to other platforms, since generally `SHLIBEXT=LTDL_SHLIB_EXT=CMAKE_SHARED_LIBRARY_SUFFIX` and `LLVM_PLUGIN_EXT=CMAKE_SHARED_LIBRARY_SUFFIX` on every platform except AIX.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D101412