Improve converted names for arithmetic operators

If `symbol->label` is used on names like `+`, you end up with `label___12345`, which is very unhelpful. Maybe we could incorporate special-case names for certain common symbols, depending on what the nasm specification allows.

https://github.com/cmsc430/www/blob/7f55e4f95b0cf4ab66430acd07ae2ecf37215d63/langs/a86/ast.rkt#L400-L421