RFR: 8221456: nmethod::make_unloaded() clears _method member too early

[Per Liden]

nmethod::make_unloaded() clears the _method member too early, before 
passing the nmethod to 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.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221456
Webrev: http://cr.openjdk.java.net/~pliden/8221456/webrev.0

/Per