xtensa: fix booting secondary cores on the dummy thread

When secondary cores are booted, they use the dummy thread and
the IRQ stack until they switch over to a real thread. Therefore
dummy threads shouldn't be skipped when cohering outgoing thread
stack, only threads with zero stack size should be skipped.

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