mpy-cross/mpconfigport: Explicitly disable native code loading.

This commit makes use of the new native code loading changes to provide
a test bed for a target with the native compiler framework available but
without no code loading capabilities.

mpy-cross already fully disables native code loading, however those
changes do not really propagate anywhere outside `py/persistentcode`.
Recent changes in the code loading framework actually touch the core
code in a few more places, but no default CI target is configured to run
with the new native code loading defines switched off.

Given that mpy-cross doesn't really need to load code anyway, it is a
good target for setting up a configuration that goes a bit deeper when
it comes to disabling code loading.  Since mpy-cross is built several
times during the CI process, it can be repurposed as an early warning
system for issues related to these new changes.  Normal operation should
not be affected in any way.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

mpy-cross/mpconfigport.h

@@ -28,6 +28,7 @@
 
 #define MICROPY_ALLOC_PATH_MAX      (PATH_MAX)
 #define MICROPY_PERSISTENT_CODE_LOAD (0)
+#define MICROPY_PERSISTENT_CODE_LOAD_NATIVE (0)
 #define MICROPY_PERSISTENT_CODE_SAVE (1)
 
 #ifndef MICROPY_PERSISTENT_CODE_SAVE_FILE