Historically, assemblers have been really dumb, so ABI is not a thing they'd track, especially as... I don't think they know what functions are? So while they can notice call/ret, they have no knowledge of a label being a jump or call target per-se, do they?
So you'd need an assembly-like language to encode this sort of information.
The short version of it is that assemblers these days are rarely - if ever - just a zero-context stream of machine instructions. There is far more, some of it actually required.
It's interesting how blurry the line is between a good, full-featured assembler and a crappy compiler!
Except you want a "macro" which:
- saves only the registers you touched
- which are callee-saved
- according to the ABI you're targeting
And you really only want that for functions, because... that's where ABIs come into play.
It's a long time since I tangled with x86 assembler; but as I recall, ENTER and LEAVE were specifically for functions, and I'm not aware of any other use for them.