libretro-beetle-sgx: add version 20180512.

Libretro is a simple but powerful development interface that allows for the
easy creation of emulators, games and multimedia applications that can plug
straight into any libretro-compatible frontend. This development interface is
open to others so that they can run these pluggable emulator and game cores
also in their own programs or devices.

Beetle/Mednafen SGX is a NEC PC Engine SuperGrafx video game system emulator
that can be used as a libretro core.

(nia)