Refactor util/mknum.pl for clearer separation of functionality

Rewrite util/mknum.pl to become cleaner, and to use the separate
generic C header parsing module, as well as the separate ordinals
manipulation module.
Adapt the build files.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7191)