RFR: 8214476: ZGC: Build ZGC by default

[Per Liden]

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