add alternate 6 button mappings for libretro

add alternate 6 button mappings for libretro
- retropad to snes style 6 button gamepad (YXL1/BAR1 = XYZ/ABC)
- retropad to madcatz style 6 button gamepad (YXR1/BAR2 = XYZ/ABC)

I asked @twinaphex if this was the correct since I was thinking into making these core options and he agreed, hopefully this can be merged so I don't have to re-add this everytime.

This responds to this request:
http://www.libretro.com/forums/viewtopic.php?f=2&t=2318