libretro: Make "Cache ISO in RAM" work again on desktops

Since b05d6171d4, the RAM cache is only used when the platform reports
SYSPROP_ENOUGH_RAM_FOR_FULL_ISO, which the libretro core never did, so
the ppsspp_cache_iso option silently did nothing. Report it on 64-bit
desktops, like the standalone app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libretro/libretro.cpp

@@ -2099,6 +2099,13 @@ bool System_GetPropertyBool(SystemProperty prop) {
       return (environ_cb(RETRO_ENVIRONMENT_GET_JIT_CAPABLE, &can_jit) && can_jit);
 #else
       return true;
+#endif
+   case SYSPROP_ENOUGH_RAM_FOR_FULL_ISO:
+      // Desktops only, like the standalone app.
+#if PPSSPP_ARCH(64BIT) && !PPSSPP_PLATFORM(ANDROID) && !PPSSPP_PLATFORM(IOS) && !PPSSPP_PLATFORM(SWITCH) && !PPSSPP_PLATFORM(UWP)
+      return true;
+#else
+      return false;
 #endif
    default:
       return false;