VMS build: better treatment of .S -> .obj compilation

It turned out that .S files aren't to be treated as lightly as I
thought.  They need to go through a preprocessing step, which .s files
don't need to.

Corrects #7703

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7889)