Implements native standard layout for Linux gamepads by using the kernel-provided button IDs, thereby expanding
support to gamepads not listed in gamecontrollerdb.txt.
Linux's docs: https://www.kernel.org/doc/Documentation/input/gamepad.txt
SDL2's source: https://fossies.org/linux/SDL2/src/joystick/linux/SDL_sysjoystick.c#l_1740
Note that I am NOT 100% convinced about the X/Y swap between Xbox and PlayStation controllers - the Xbox
compatible pad I have however does have BTN_NORTH and BTN_WEST swapped (and thus BTN_X and BTN_Y
assigned right), which confirms SDL's logic and opposes the kernel docs.
Tested with this gamepad: "20d6:2802 BDA Xbox ONE Core controller", label says "PowerA Model 1508491-02" - even
after clearing out gamecontrollerdb.txt, examples/gamepad shows a 100% correct mapping.
Closes#2052
