ARM 状态到 Thumb切换的例程，为了增强系统的灵活性以及提高系统的整体性能经常需要使用16位的Thumb指令,所以需要在ARM和Thumb状态之间来切换。-ARM state to Thumb switch routine, in order to enhance the flexibility of the system and to improve the overall performance of the system often need to use 16-bit Thumb instructions, there is a need in the ARM and Thumb state to switch between.