Accuracy-focused multiplayer expansion for GBA_MiSTer

Let's get it out there! Wanna play GBA couch co-op on your MiSTer? This is the core for you c:

Both cores are built from the upstream project's accuracy branch for those original fixes among others added later by myself.

Changelog: 2P core

New: multiplayer over the link cable: two players on one MiSTer (side-by-side), or link to a real GBA via SNAC.
- Verified in-game with:
  - Apotris multiplayer (also written by yours truly!)
  - the gba-link-connection demos, as well as
  - various OEM retail games
New display options: show Both players, or fullscreen either player (game keeps running for both); optional separator line.
New audio options: hear Player 1, Player 2, a mix — or split P1-left / P2-right for headphone couch play.
2P build won't show savestate/cheat options that don't exist on it to save core space
Link-port (SIO) register fixes for untested/unsupported SNAC link cable features
- WIP: be careful of 5V vs 3.3V if using real GBA hardware with your GB EXT SNAC extensions!! (optional, rare)

RTC fixed for all clock carts (Pokémon Ruby/Sapphire/Emerald, Boktai…):
- a single game's compatibility hack is now ONLY active on that game only
- this previously always-on hack silently broke clock-chip detection for every other RTC game such as custom Pokémon hacks
The in-game clock now sets itself from your system time on first boot instead of a fixed 2009 date
- Note: Existing saves without core-- stored RTC data will see a one-time time jump to adjust