Use gethostbyname_r if available

Fixes #7228

The function BIO_get_host_ip uses gethostbyname, which is not thread safe
and hence we grab a lock. In multi-threaded applications, this lock sometimes
causes performance bottlenecks.
This patch uses the function gethostbyname_r (thread safe version), when
available.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7250)