[libretro] Implemented input bitmasks

[keithbowes]

Input bitmasks were added to RetroArch a couple years ago as a more efficient way of determining which buttons are pressed. Instead of calling a function for each button, a core needs only to call a function once to get a bitmask of all buttons pressed. The old way remains for frontends that don't support input bitmasks.

[notaz]

Hmm can an usable libretro core still be built from this repo? I think almost everyone has switched to the @irixxxx's fork: https://github.com/irixxxx/picodrive

[irixxxx]

this change is already in my fork.