Review request: 8055856: checkdeps build target doesn't work for cross-compilation builds

[Mandy Chung]

Erik, Magnus,

This is much easier than I have thought.  I really like this new build.
I have separated out Gendata-jdk.dev.gmk and removed the modules-xml
target completely.

Webrev at:
   http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8055856/webrev.01/

Mandy