[libcpu][arm] Fix compilation warning
[libcpu][arm] Fix compilation warning

nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread