1.Forward nxsched_process_cpuload to nxsched_process_cpuload_ticks directly
2.Define the dummy nxsched_process_cpuload_ticks when CPULOAD isn't enabled
3.Remove the weak attribute from nxsched_process_cpuload_ticks
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>