These files are identical to the files in the expat git repository.

So it looks like I need to submit the fix and get it accepted to that repository first.  And then at some point cPython will pick up the change.

I can still provide diffs if desired, but based on Victor's comment, they would not be applied.

Historical Note:
That VMS specific code is only needed for building on VAX/VMS 5.5-1 and earlier with the long obsolete VAX C.  VAX/VMS 5.5-1 was released in June 1992.

VAX/VMS 5.5-2 was released November 1992, and around that time a new DEC C compiler release appeared that no longer needed that hack, but used it as the default mode for backwards compatibility.  That default mode is not compatible with many Open Source projects.