py/persistentcode: Introduce MICROPY_PERSISTENT_CODE_SAVE_FILE option.

This should be enabled when the mp_raw_code_save_file function is needed.

It is enabled for mpy-cross, and a check for defined(__APPLE__) is added to
cover Mac M1 systems.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 1df2e8f..518bddd 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -283,6 +283,11 @@
 #define MICROPY_PERSISTENT_CODE_SAVE (0)
 #endif
 
+// Whether to support saving persistent code to a file via mp_raw_code_save_file
+#ifndef MICROPY_PERSISTENT_CODE_SAVE_FILE
+#define MICROPY_PERSISTENT_CODE_SAVE_FILE (0)
+#endif
+
 // Whether generated code can persist independently of the VM/runtime instance
 // This is enabled automatically when needed by other features
 #ifndef MICROPY_PERSISTENT_CODE