It is likely the solution is really easy.
Just check that the specified production hardware target is correctly configured for your target. Make sure that all the integer sizes agree with the C compiler you will use for your target. Also make sure the checkbox "Support long long" is appropriately set.
MATLAB Coder also has the same configurations for setting production target integer sizes and allowing use of the long long type.
When an integer or fixed-point type used in your model is bigger than the longest integer type that your target C compiler provides, then multi-word types will be used in the generated code.
These multi-word typedefs will have names like int64m_T, int96m_T, int128m_T, etc.
The appearance of int64m_T indicates that as configured for on your Simulink model or MATLAB Coder project, the biggest integer available for the C compiler is less than 64 bits.