VMS: Copy DECC inclusion epi- and prologues to internals

Because many of our test programs use internal headers, we need to make
sure they know how, exactly, to mangle the symbols.  So far, we've done
so by specifying it in the affected test programs, but as things change,
that will develop into a goose chase.  Better then to declare once and
for all how symbols belonging in our libraries are meant to be treated,
internally as well as publically.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3259)