RFR: 8219638: ZGC: Free ZNMethodDataOops using GlobalCounter

[Erik Österlund]

Hi,

Today, the ZNMethodDataOops use a custom deferred freeing mechanism to 
handle the case where C1 performs code patching that generates more 
immediate oops, while the GC is concurrently walking the code cache for 
class unloading.
It would be nice to use GlobalCounter instead, which is a more 
standardized way of doing it.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8219638/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8219638

Thanks,
/Erik