Fix issue where loaded global modules aren't freed

Fix issue where loaded global modules aren't freed

Globally loaded modules would previously call LoadLibrary on Windows, but never FreeLibrary. This would result in said library keeping an internal reference counter of 1 until the application exited its entry-point, resulting in possible shutdown issues.

This commit makes sure that we correctly free modules (and unload them in the platform-specific APIs) whenever Mono itself is shutting down, ensuring the expected result of Mono-loaded libraries being correctly freed.