community/libretro-{nestopia,mgba,desmume}: uniform pkgver to 0_gitYYYYMMDD
libretro cores are usually never versioned as they are rolling-released. Previous packager was using a version + the git tag but it does not make sense as RetroArch team does not release cores.
Unfortunately, using the new 0_gitYYYYMMDD syntax will not be detected as newer so I've been told to wait before 3.14 gets released so we can finally do this change.
This change uniform every libretro cores to this numbering scheme.