games/libretro-*: make portversions go forwards

• games/libretro-desmume2015: restore accidentally deleted PORTREVISION
• games/libretro-emux: distversion now reflects date of last commit, but grabs the same git commit. Adjust PORTEPOCH.
• games/libretro-snes9x2005: upstream uses versions, but didn't release a new one since 2013; use a git-describe style inbetween version.
• games/libretro-snes9x2010: restore accidentally dropped PORTEPOCH

Reported by: portmgr