RFR: 8214476: ZGC: Build ZGC by default

[Magnus Ihse Bursie]

Looks good to me. 

/Magnus

> 29 nov. 2018 kl. 13:35 skrev Per Liden <per.liden at oracle.com>:
> 
> At one point, it was decided that experimental VM features (such as ZGC) should be not be built by default. This policy was later changed/abandoned, so we should enable building of ZGC by default.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8214476
> Webrev: http://cr.openjdk.java.net/~pliden/8214476/webrev.0
> 
> /Per