Patch from Michael Hohnbaum

This adds a hook, sched_balance_exec(), to the exec code, to make it
place the exec'ed task on the least loaded queue. We have less state
to move at exec time than fork time, so this is the cheapest point
to cross-node migrate. Experience in Dynix/PTX and testing on Linux
has confirmed that this is the cheapest time to move tasks between nodes.

It also macro-wraps changes to nr_running, to allow us to keep track of
per-node nr_running as well. Again, no impact on non-NUMA machines.