RFR: 8219718: ZGC: Make nmethod entry barriers and nmethod::is_unloading use ZNMethodDataOops

[Erik Österlund]

Hi,

Accessing ZNMethodDataOops outside of concurrent code cache iteration 
used to be unsafe. It is now safe. Therefore, we should utilize this in 
the nmethod entry barriers and nmethod::is_unloading.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8219718/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8219718

Thanks,
/Erik