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

[Per Liden]

Looks good!

/Per

On 02/26/2019 01:23 PM, Erik Österlund wrote:
> 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