Add "weak" declarations of symbols used in safestack.h and lhash.h

Only for SunCC for now.

It turns out that some compilers to generate external variants of
unused static inline functions, and if they use other external
symbols, those need to be present as well.  If you then happen to
include one of safestack.h or lhash.h without linking with libcrypto,
the build fails.

Fixes #6912

Signed-off-by: Matthias Kraft <Matthias.Kraft@softwareag.com>

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8087)

(cherry picked from commit 6638b221)