Another classic trait of trolls, is a tremendous regenerative ability. Sometimes to the point where only fire can kill them.

So I'd suggest trolls create a chemical in their body that boosts their regenerative abilities, and any damage will increase the levels of this chemical. Sadly (for the troll) this chemical reacts with UV light from the sun, and hardens. Creating the resemblance of turning to stone.

That could be an explanation for why only sunlight, and not other kinds of light (fires usually don't emit UV light) it also explains why they haven't evolved out of such a vulnerability, as they would loose their regenerative ability.

It can be adjusted from only their skin turning to stone, and at night they can break free (loosing their skin, but it'll regenerate almost immediately) all the way to turning completely to stone (UV light can penetrate pretty deep, that's why we use sunscreen to not get cancer)