VMS: only use the high precision on VMS v8.4 and up

It simply isn't available on older versions.

Issue submitted by Mark Daniels

Fixes #7229

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7230)