Bug fix: an interrupt may return to the end of function "rt_hw_context_switch_to"

[hwang1024]

[
Description:
In cortex-M uC, there is a tail-chain mechanism to fasen interrupt/exception response,
from simulation tool: KEIL V4, it shows that a mismatch of stack restore occours if: an interrupt/exception happens at the moment that returning from previous interrupt, and MSP will be restored instead of task's PSP, and it'll cause a return to the end of function: rt_hw_context_switch_to, causes a hard fault in the end.

Purpose:
To solve the issue in case an interrupt service is tail-chained by another interrupt, and return back to function rt_hw_context_switch_to

Tested Enviroment:
KEIL V4.03 with similator / STM32F103RC, with project "rt-thread-v4.0.2\rt-thread\bsp\stm32\stm32f103-yf-ufun\project.uvproj"
1> debug(simulator) => run => set breakpoint at end of function "void SysTick_Handler(void)"
2> peripherals => core peripherals => NVIC (nested vectored...) =>index 53 USART1 (Set pending bit manually)
3> run & check...
]

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[mysterywolf]

related：
其他的Cortex M4 M7 M0是否也存在同样的问题？
GCC IAR下是否存在同样的问题？
#4728 此pr也提到了一些context_switch_to函数的问题。

[cndabai]

楼主指的是在返回task时，还是在使用MSP吗？可以详细讲一下复现流程嘛，这边按照上面步骤一直没有复现

[hwang1024]

可能和KEIL 版本有关系, 用uV4.03版本模拟仿真每次都能复现，不用RTOS只使用MSP就没问题，会让开发者觉得系统不稳定

[hwang1024]

[mysterywolf]

@cndabai 用Keil5重复上述同样操作，不用keil4，测试例程可以选择stm32-blue-pill，这个bsp已经设置好仿真参数了，开箱就能仿真。

[cndabai]

keil5并没有复现，这边LR是0xFFFFFFFD，正常，返回后是PSP
一会下个keil4再试一下
楼主了解这个BUG产生的原因吗？有没有在其他环境下试一下，是模拟器问题吗

[mysterywolf]

如果Keil5没有 但是Keil4出现，高度怀疑是keil自己的原因。

[mysterywolf]

@hwang1024 建议升级到keil5再尝试复现一下，如果是keil本身的bug，我们是没有办法替他的bug来修改程序的

[cndabai]

keil4.10也没遇到。不知道是不是姿势不对

[Guozhanxin]

If it can't be reproduced, then temporarily close the current pr?

[mysterywolf]

如果keil5复现不出来这个问题的话，那么八成这个问题是由于keil4自己的问题导致的，我们的代码不会对keil4自己的问题负责。