Prefetch blocks; don't make clean everything

With blocks being prefetched I don't gain any better results but a whole
bunch of L1d cache misses from cachegrind - the same amount as without
prefetch. I'll investigate...

The 'clean' target is responsible for cleaning the current directory and
affiliates (like asm/ is to the toplevel directory) only. For descending
into independent subdirectories (as naive/ would be to the toplevel
directory), there now is 'deepcl' with the deepclean_targets variable.