Add gamepad mappings for GameCube USB adapters

[999sian]

• I confirm that I am the author of this code and release it to the SDL project under the zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

Adds missing gamepad database mappings in src/joystick/SDL_gamepad_db.h for DragonRise / Mayflash GameCube controller adapters:

• Added macOS mapping for Mayflash 2-port adapter (0079:1843), matching the existing Windows and Linux mappings.
• Added Windows (DInput), macOS, and Linux mappings for clone 4-port / 2-port GameCube adapters (0079:1846), matching existing 0079:1843 / 0079:1844 button and axis layouts.

Verified canonical ordering with src/joystick/sort_controllers.py.

[slouken]

Merged, thanks!