smp: fix a race when starting / resuming multiple CPUs

cpu_start_fn is global, it's used by the initiator CPU to start or
resume secondary CPUs. However it's possible, that the initiator CPU
goes ahead and starts a second secondary CPU before the first one has
finished using the object. Fix this by creating a local copy of the
global object.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>