RFR: JDK-8244009 Separate -Xdoclint options in CompileJavaModules.gmk

[Magnus Ihse Bursie]

Many modules get specialized -Xdoclint flags in CompileJavaModules.gmk.  
Long term, unless these are cleaned up and normalized, we might want to 
think about creating a better framework for handling those (similar to 
the DISABLE_WARNINGS_*), but as a short-term solution we could just 
separate them out to a specialized variable, to better see what "real" 
arguments to javac modules are passing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8244009
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8244009-separate-doclint/webrev.01

/Magnus