RFR: JDK-8073634: Improve clean targets

[Erik Joelsson]

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/

/Erik