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