You can process almost any powdered food item flour into something resembling bread, if you expend enough effort on it. You can make bread with cricket flour. Even meatloaf could be considered a type of "bread", if you massage the definitions long enough.
But I'd guess that Tibetans mainly eat barley bread. Rye and barley aren't as good as wheat is for making leavened breads, but they can make a loaf that isn't entirely brick-like, if the baker keeps a good sourdough starter and it gets a lot more rise time.
In all seriousness, you can make "bread" from damn near any grain if you're persistent enough and grains of various types (and small fruits, i.e. berries) can grow basically anywhere. It's large fruit and vegetables that tend to require easy mode to grow (which makes sense since they spend proportionally more resources growing that the fruit they produce). I'm not sure what's native to tibet but I assume they have some sort of wheat-like grass that can be farmed and provides grain for bread.
That CCP is just so nice and helpful!