AP_HAL_ChibiOS: introduce get_HAL_mutable() to complement get_HAL()

Returns a mutable reference to the same HAL for certain purposes where
the HAL needs to be mutated to avoid UB problems with casting away const
and to make the fact that mutation is happening obvious.

libraries/AP_HAL_ChibiOS/HAL_ChibiOS_Class.cpp

@@ -344,8 +344,13 @@ void HAL_ChibiOS::run(int argc, char * const argv[], Callbacks* callbacks) const
     main_loop();
 }
 
+static HAL_ChibiOS hal_chibios;
+
 const AP_HAL::HAL& AP_HAL::get_HAL() {
-    static const HAL_ChibiOS hal_chibios;
+    return hal_chibios;
+}
+
+AP_HAL::HAL& AP_HAL::get_HAL_mutable() {
     return hal_chibios;
 }