RFC: Update x86 psABI to support shadow stac

[Shanbhogue, Vedvyas]

>> Is there a requirement that the return address is popped from the same 
>> stack location where it was pushed by the call instruction?  Or could 
>> you return with an elevated stack pointer if you copied the address first?

> Stack location isn't checked.  Only the popped return address is checked.
> Vedvyas can confirm it.

Yes. That’s correct.