RFR: JDK-8203945: Cleanup nashorn build

[Tim Bell]

Erik:

> 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

Looks good.

/Tim