rand/rand_unix.c: bypass DSO_global_lookup on ELF systems.

If built with no-dso, syscall_random remains "blind" to getentropy.
Since it's possible to detect symbol availability on ELF-based systems
without involving DSO module, bypass it.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6436)