[LowerGlobalDtors] Skip __cxa_atexit call completely when arg0 is unused (#68758)

In emscripten we have a build mode (the default actually) where the
runtime never exits and therefore `__cxa_atexit` is a dummy/stub
function that does nothing. In this case we would like to be able
completely DCE any otherwise-unused global dtor functions.

Fixes: https://github.com/emscripten-core/emscripten/issues/19993