Merge branch 'libretro' into libretro-reset

This effectively resets libretro git to upstream and syncs both repos.
Everything in platform/libretro should be retained as I imported that in
my git verbatim, but some core changes may be lost and will have to be
redone. I've checked that picodrive_libretro.so builds and works on x86,
and Android also builds for all archs. I've even bothered to try it
under msvc where it's building for me too.

With this you also get the benefits from significant accuracy improvement
work from 1.92.