Make openVMS seeding less dependent of OpenVMS version

Make openVMS seeding less dependent of OpenVMS version

SYS$GETTIM_PREC is a very new function, only available on OpenVMS v8.4.
OpenSSL binaries built on OpenVMS v8.4 become unusable on older OpenVM
versions, but building for the older CRTL version will make the high
precision time functions unavailable.

Tests have shown that on Alpha and Itanium, the time update granularity
between SYS$GETTIM and SYS$GETTIM_PREC is marginal, so the former plus
a sequence number turns out to be better to guarantee a unique nonce.

Fixes #18727

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18731)