I'm of the opinion that parts of the northeast United States would have survived. They are mountainous, self-sufficient if in small populations, due to the prevalence of agriculture, and, in winter, can definitely get colder than Iceland.
However, they cannot be cut of as easily, as they have no water routes barring entry.
Additionally, I would like to make a case for parts of China.
http://www.thaigoodview.com/library/studentshow/2549/m6-4/no12-14-45/china/picture/china_lulc.jpgThis is a map of its geography.
http://mtr1600china.files.wordpress.com/2012/08/china-physical-map.gifThis is a map of some cities.
http://www.chinatouristmaps.com/assets/images/chinamaps/china-temperature.jpgThis a map of temperatures.
Though city centers are unlikely to have survived (...unless the government really, REALLY cracked down, in a terrible manner), it's likely that Xinjiang, the region lining Mongolia, and northeast China would have pockets of survivors, in addition to the Tibetan plateau, as was already mentioned. Cities along the Gobi desert might also be okay.
China is also huge, and not very developed outside the city centers, so smaller, far-flung towns (and there are a lot of isolated villages still) might not be impacted for a decent amount of time.
Likewise, Russia definitely has yearlong, and somewhat isolated, populations in Siberia.
As for Mongolia, the nomadic steppe culture has not entirely died, and is definitely not far from the minds of the inhabitants. I don't see why they wouldn't take it up again, particularly in a life-and-death situation where societal status no longer matters.
Regarding the use of cold to exterminate trolls, I believe it has to do with them dying from exposure, as their nests and burrows are what must be compromised. Following this, heat would likely be just as able to exterminate trolls, provided it was extreme enough. Given that, desert areas are, in my opinion, also likely to have survivors.
Areas where trolls cannot easily hide from the sun, and cannot easily build and find shelter - such as the prairies and plains of the United States, or, as mentioned above, deserts - might be good.
Jungle's toast, though. No matter how isolated the groups may be.
Just as an aside, we have no idea what the rate of immunity is for non-Caucasian groups.