I'm not sure you could even make the suit tight enough to show his abs like that. I'm sure it's mostly his own physique, but with some enhancements built into the fabric for better definition.
Yeah, those abs are clearly being enhanced a little bit. Although Cavill clearly bulked up quite a bit for the movie, the suit just doesn't look thin
enough for those muscles and veins to be entirely his.
It's not like he's wearing a wet T-shirt or something.