RFR: JDK-8073634: Improve clean targets

[Tim Bell]

Erik:

> Our top level clean targets need some improvement.
>
> * There is no clean-docs
> * dist-clean does not remove everything configure creates on all 
> platforms
>
> For the latter, moving a lot of the files generated by configure into 
> a separate sub dir of the output dir helped. It's named 
> configure-support as to mirror make-support. The difference between 
> the two is who is responsible for the files inside them and thus which 
> target will clean them.
>
> I also made the clean targets work better with LOG=debug, so you can 
> actually see the rm commands.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8073634
> Webrev: http://cr.openjdk.java.net/~erikj/8073634/webrev.01/

Very nice.  Looks good to me.

Tim