It probably doesn't make a lot of sense to analyze an interjection according to typical grammatical rules. These sorts of interjections, or exclamations typically are not used as parts of complete sentences.
For example, how would you analyze the exclamation, "whatever", as in:
Person A: You should be more careful.
Person B: Yeah, whatever.
It might be possible to see "whatever" as a highly abbreviated expression of the thought: "Whatever you say, you can't make me care." But realistically, that is not what's going through the speaker's head. It's really not an abbreviation, though historically it may have started out that way.
The most compelling argument (maybe the only compelling argument) for the correct spelling on this interjection would probably be based on the accepted spelling of similar expressions.
For fuck/s/'s/s' sake closely resembles
For God's sake
For Christ's sake
Which would favor "for fuck's sake".
But there's also
For goodness sake
This seems to suggest that "for fucks sake" might be correct, or possibly even "for fuck sake." I know that I, for one, do not pronounce two distinct s's. Perhaps the only s I'm pronouncing is the s in sake.