They could already do that. They sign and encrypt the (plaintext) code segment you submit so they could easily insert a JMP to their own payload at the head of your code any time
They could also just have the kernel do whatever it wants to your program, because they control that too. If you are worried that Apple might tamper with your binaries if given the chance, using an iOS device is pure folly, because they outright control much more important parts of your system (the kernel, the UI libraries, the RNG…).
