Write vx_spawn_tasks_cluster

This scheduling logic tries to evenly distribute warps across *all* cores,
instead of trying to fill up the first cores as much as possible.  This scheme
is necessary for the intra-cluster cores which are assumed to have equal
workloads distributed.

kernel/include/vx_spawn.h

@@ -50,6 +50,7 @@ void vx_wspawn_wait();
 void vx_spawn_kernel(context_t * ctx, vx_spawn_kernel_cb callback, void * arg);
 
 void vx_spawn_tasks(int num_tasks, vx_spawn_tasks_cb callback, void * arg);
+void vx_spawn_tasks_cluster(int num_tasks, vx_spawn_tasks_cb callback, void * arg);
 
 void vx_serial(vx_serial_cb callback, void * arg);