backport mame2003 libretro path logic (PR from rrifonas)

[markwkidd]

This patch comes from gbatemp user riffonas, who used mame2003 as a model. riffonas is working with the Atari Flashback Legends which is an ultra low-power SoC that uses a custom libretro frontend.

This Atari libretro frontend doesn't properly pass paths to the core, so having the legacy/backup path logic from mame2003 lets people with the Flashback Legends system drop in a different core and still be able to load and save as needed.