RFR: 8214476: ZGC: Build ZGC by default

[Aleksey Shipilev]

On 11/29/18 1:35 PM, Per Liden wrote:
> 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

Finally! Looks good!

-Aleksey