RFR: JDK-8245287 Start using ModuleWrapper for gensrc as well

[Magnus Ihse Bursie]

All other phases are using ModuleWrapper.gmk, except for gensrc. We 
should move that too over to the ModuleWrapper way, so all phases are 
treated equally.

Bug: https://bugs.openjdk.java.net/browse/JDK-8245287
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8245287-use-ModuleWrapper-for-gensrc/webrev.01

/Magnus