This PR modifies the Emscripten joystick backend to detect the user's OS and store its ID in the GUID, because different OSes might need different mappings for the same controllers.
I'm not sure if different browsers on the same OS can also have different mappings, but if they can, browser detection can be added to the GUID too if needed.
This PR also makes the GUID use `SDL_HARDWARE_BUS_USB` instead of `SDL_HARDWARE_BUS_UNKNOWN`, similarly to how Android and MFI backends always use `SDL_HARDWARE_BUS_BLUETOOTH` and GameInput, XInput, and RawInput backends always use `SDL_HARDWARE_BUS_USB`.
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