Add core options to default to 2 or 6 buttons controllers

[bslenul]

Add core options to let the user chose if the joypads should be 2 or 6 buttons by default, so if you want to play a 6 buttons game, you don't have to press the "Mode Switch" every time you boot it. It's also very useful for SNES-like controllers without a "L2" button, previously you had to remap the "Mode Switch" button, return to game and press it, then remap it back, super annoying...

[negativeExponent]

https://github.com/libretro/beetle-supergrafx-libretro/issues/132

[anzz1]

Which one of the 5 games released for SuperGrafx uses 6 buttons?

[bslenul]

The core is compatible with PCE/PCE-CD games.