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

Fixes #8487
Amends #7230

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8488)

(cherry picked from commit 355417eb)