2.11.10. __rt_stack_postlongjmp()
This function sets sp to a correct value after a call to longjmp(). An incomplete prototype implementation in assembler code is in rt_memory.s.
This function is called with r1 containing the pre‑setjmp() value for sp.
If your memory model is used only with non stack‑checked AAPCS, you do not have to implement this function.
The function must set sp to a valid post‑longjmp() value. Therefore, in the minimal implementation where the memory model requires no special handling, you would simply set sp to the value in r1.