nmethod::make_unloaded() clears _method member too early

nmethod::make_unloaded() clears the _method member too early, before passing the nmethod to the CollectedHeap::unregister_nmethod(). This is not what happens when an nmethod is unregistered via nmethod::make_not_entrant_or_zombie(). We should align this behavior. Clearing the _method member after it has been unregistered is useful, since the GC can then print the method name/signature in logs, etc. Moving the clearing of _method until after CollectedHeap::unregister_nmethod() should be a safe and uncontroversial thing to do.

Today, ZGC can crash if -Xlog:gc+nmethod=debug is used and an nmethod is unloaded via nmethod::make_unloaded(), because it tries to log the name of the method.