RFR: JDK-8203945: Cleanup nashorn build

[Erik Joelsson]

The nashorn module requires some special handling when being built. 
Because of this, it's not currently compiled like the other modules, in 
CompileJavaModules.gmk, but instead in a separate BuildNashorn.gmk. 
While nashorn still needs some special treatment, because of the nasgen 
tool, this can be reduced quite a bit.

This patch reduces (and renames) BuildNashorn.gmk to only be about 
running nasgen and include that file conditionally from 
CompileJavaModules.gmk. That way the nashorn classes will be compiled 
using the same configuration as any other module.

Bug: https://bugs.openjdk.java.net/browse/JDK-8203945

Webrev: http://cr.openjdk.java.net/~erikj/8203945/webrev.01/index.html

/Erik