Add cheat support

The attached commits implement support for all cheat systems supported by VBA-M. The core now relies on RetroArch for content loading.

This pull request does not link or call functions that require zlib support:

    [higor@whiterice libretro]$ objdump -tC vbam_libretro.so | egrep -i 'gz|zl|flate'
    00000000000dbea0 g     F .text	0000000000000060              utilIsGzipFile(char const*)