From: Ingo Molnar <mingo@elte.hu>

Implement balancing during clone().  It does the following things:

- introduces SD_BALANCE_CLONE that can serve as a tool for an
  architecture to limit the search-idlest-CPU scope on clone().
  E.g. the 512-CPU systems should rather not enable this.

- uses the highest sd for the imbalance_pct, not this_rq (which didnt
  make sense).

- unifies balance-on-exec and balance-on-clone via the find_idlest_cpu()
  function. Gets rid of sched_best_cpu() which was still a bit
  inconsistent IMO, it used 'min_load < load' as a condition for
  balancing - while a more correct approach would be to use half of the
  imbalance_pct, like passive balancing does.

- the patch also reintroduces the possibility to do SD_BALANCE_EXEC on
  SMP systems, and activates it - to get testing.

- NOTE: there's one thing in this patch that is slightly unclean: i
  introduced wake_up_forked_thread. I did this to make it easier to get
  rid of this patch later (wake_up_forked_process() has lots of
  dependencies in various architectures). If this capability remains in
  the kernel then i'll clean it up and introduce one function for
  wake_up_forked_process/thread.

- NOTE2: i added the SD_BALANCE_CLONE flag to the NUMA CPU template too.
  Some NUMA architectures probably want to disable this.