fw: iec: Fix warning about references to static mut

References to static mut quickly lead to undefined behavior.  Adjust the
code such that no direct references to static mut variables are created.

Firmware-IEC/src-rust/matiec/ext.rs

@@ -22,13 +22,13 @@ macro_rules! io_tags_def {
         paste::paste! {
             $(
                 #[export_name = concat!("__", stringify!($IO))]
-                static mut [<$IO _ADDR>]: *const $Ty = unsafe { &$IO };
+                static mut [<$IO _ADDR>]: *const $Ty = unsafe { ::core::ptr::addr_of!($IO) };
                 pub static mut $IO: $Ty = 0;
             )*
 
             pub unsafe fn all_mut<'a>() -> [&'a mut $Ty; $N] {
                 [
-                    $(&mut $IO,)*
+                    $(&mut *::core::ptr::addr_of_mut!($IO),)*
                 ]
             }
         }