Fix interrupt handling

## Scenery

There is currently an implementation of Interrupt handling:
- [include/libgba-cpp/arch/cpu/interrupts.h](https://github.com/JPTIZ/libgba-cpp/blob/master/include/libgba-cpp/arch/cpu/interrupts.h);
- [include/libgba-cpp/arch/cpu/interrupts.cpp](https://github.com/JPTIZ/libgba-cpp/blob/master/src/arch/cpu/interrupts.cpp).

**TODO**: describe what is not working.