rts: Ensure that WinIO flag is set when --io-manager=auto

As noted in #25838, previously `selectIOManager` failed to set
`rts_IOManagerIsWin32Native` in its `IO_MNGR_FLAG_AUTO`. This meant
that the MIO path was taken when WinIO was supposedly selected,
resulting in chaos.

Fixes #25838.

rts/IOManager.c

@@ -233,7 +233,6 @@ void selectIOManager(void)
 #if defined(IOMGR_ENABLED_WINIO)
         case IO_MNGR_FLAG_WINIO:
             iomgr_type = IO_MANAGER_WINIO;
-            rts_IOManagerIsWin32Native = true;
             break;
 #endif
 
@@ -246,6 +245,10 @@ void selectIOManager(void)
         default:
           barf("selectIOManager: %d", RtsFlags.MiscFlags.ioManager);
     }
+
+#if defined(mingw32_HOST_OS)
+    rts_IOManagerIsWin32Native = iomgr_type == IO_MANAGER_WINIO;
+#endif
 }