Restore the use of LDCMD when linking applications

It is a hack, but it existed in the recently removed Makefile.shared,
and its use is documented in fuzz/README.md, so we cannot drop it now.

Fixes https://github.com/google/oss-fuzz/issues/1037



Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4947)