I was hasty in my reply, because you need to store at least one thing to make
it work: the return address you replaced with yours. You can't keep it r10
because it's a volatile register. You can't use the stack because you have
none available. You have to use a thread-local variable and to make that
reentrant, you'd need to have a linked list of return addresses allocated
using the heap. So it's possible... just terribly cumbersome.