cmelf: Add script to import the FreeBSD ELF headers

These headers will be used to provide the ELF parsing code on all host
operating systems (including those that don't have an ELF header, e.g.
macOS).  This will also allow removing various OS-dependent #ifdefs from
the cmELF code.  Add a script that was used to automate this import.

Co-authored-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>