VMS: Change debug linking method to generate a separate Debug Symbol File

That makes it possible to run images without automagically ending up
in a debug session, while still being able to debug when required.

All .DSF files must reside in the same directory to be useful.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2947)