This might be a tangent, but why would STL prevent cross-platform development? STL is literally the standard guaranteed to exist in all (compliant) C++ compilers.. I get that you can’t expect the the same STL performance on all platforms, though..
More on-topic, though: Are you sure that unordered_map performance is actually an issue at all for you?
If you do measurements, please share them though, I would also be interested..