hadrian: Offer way to clean everything compiled with stage1 upwards

Often, I do some tinkering and then find that I need to rebuild all libraries compiled by the stage1 compiler, perhaps because I did some changes to an optimisation. At the time of writing, I don't know of any other way than to completely nuke the build root and rebuild the whole stage1 compiler from scratch, too. And then I have wait an additional 5 minutes before I see the results of my change, which often is just another crash quite near where it crashed last time.

Is there a way to tell hadrian to invalidate everything under _build/stage{1,2}? Simply nuking those sub-folders results in a broken build root, it seems.