The enter key.
Seriously, if you really want to do such a depraved thing, you can use anything you want, but you'll have to run it through a preprocessor before you feed it to Nasm.
I think I've got one for dos that replaces "//" with a CRLF. Maybe a Perl script that uses ";" to separate lines... something else - "#"? - for comments. Not too difficult to write yourself such a utility. Makes the code harder to read and *much* harder to maintain. You do it, you deserve it!
Best,
Frank