sched: add export cpu cfs load variable

In a large and busy system, the cfs.runnable_load_avg cacheline is
touched quickly when sched entity enqueue/dequeue and load update.

A extra exported cpu cfs load can reduce the cache bouncing when
doing load balance and reduce the task migration if change is small.

Inspired-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>