RFR: 8214476: ZGC: Build ZGC by default

[Roman Kennke]

Yes! Great! Patch looks good!

Roman


> 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