Core option to rotate buttons when using MS Joypad 2 Button?

[vaguerant]

Inspired by the core option in Nestopia:

https://github.com/libretro/nestopia/blob/21e2cec7a13f0a09f493637de289e59386e2fd36/libretro/libretro_core_options.h#L266-L279

https://github.com/libretro/nestopia/blob/767200a06fc2dd8cd9532889370d40aec5290b43/libretro/libretro.cpp#L818-L826

https://github.com/libretro/nestopia/blob/767200a06fc2dd8cd9532889370d40aec5290b43/libretro/libretro.cpp#L541-L552

libretro emulators generally (always?) map two-button controllers to the RetroPad B and A buttons by default. For action games, Y and B can be more comfortable on the modern, 4-button diamond layout. Nestopia includes a core option for this setup.

While I don't think this feature is useful or sensible for the three or six button Mega Drive pads, the XE-1AP, etc., I think it would be convenient for the MS Joypad 2 Button.

Rotated SMS buttons are achievable currently by using e.g. content directory or game remaps within RetroArch, but a core option in the Input section would make this a lot more user-friendly.

Note: If this change does happen, the Core Options > Input tooltip would also need updating, as it currently reads "Configure light gun / mouse input."

Filing this here as I'm specifically suggesting a RetroArch core option, but this could also be useful upstream if desired.