Add link General Purpose mode for FFTA.

Remove code that unconditionally resets RFU on Windows in General
Purpose mode, and instead remap two values necessary for Final Fantasy
Tactics Advance.

Thanks to @JackoboLeChocobo for this information.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/gba/GBALink.cpp

@@ -708,10 +708,17 @@ void StartGPLink(uint16_t value)
         break;
 
     case GP:
-#if (defined __WIN32__ || defined _WIN32)
-        if (GetLinkMode() == LINK_RFU_IPC)
-            rfu_state = RFU_INIT;
-#endif
+        // General purpose mode, for circuit reset and Final Fantasy Tactics.
+        // Thanks to @JackoboLeChocobo for this info.
+        switch (value) {
+            case 0x8055:
+                value = 0x8052;
+                break;
+            case 0x8022:
+                value = 0x8025;
+                break;
+        }
+        UPDATE_REG(COMM_RCNT, value);
         break;
     }
 }