enable single coma for `push/pop/...` even with `--syntax=a`

[ped7g]

when the instruction has clearly only single argument (push is ok, sub is not) and there's very tiny chance of any ambiguity, enable single coma even when using --syntax=a.

This will also help with assembling sources migrated from other assemblers, which allow only push/pop arguments, but support implicit/explicit a for sub/sbc/add/xor/... so using --syntax=a will not stop push/pop from working.