Cleanup nashorn build

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.

My intention is to reduce 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.