Enhancement: Add system2x6 support (via pcsx2x6 fork)

Enhancement request: Add system2x6 support

There's a lot of active development going on over in the PCSX2 world in this new project https://github.com/PS2Homebrew-arcade/pcsx2x6, while I'm not personally familiar with how far diverged the libretro PCSX2 fork is from upstream, it might be worthwhile to cherry pick through the commits in pcsx2x6 and port them to the libretro lrps2 core.

Justification

As many people know, many of the system2x6 games are in a completely unplayable state with the Play! PS2 emulator, this could be an easy route to getting some sort of compatibility with those on the PCSX2 side, which generally performs much stronger.

This issue is primarily just to get the baseline request in place and to gauge interest, I'm sure there's lots of research to be done on this end as the PCSX2x6 fork might be leveraging APIs or abstractions that explicitly only exist in the upstream and not the diverged libretro fork.

I'd be interested in contributing to this as well, after of course, the barrier of familiarizing myself with the lrps2 core.

Start by comparing the pcsx2x6 fork with the libretro lrps2 core, focusing on commits and APIs or abstractions that may not exist in the diverged fork. Identify a compatible baseline for porting the relevant changes, then verify that system2x6 games gain compatibility without regressing existing core behavior.