Configurations/unix-Makefile.tmpl: switch to "natural" AIX shared libraries.

AIX treats its shared libraries in unique manner, by placing multiple
shared objects of different versions and bitnesses, into .a file.
So far we have been naively linking with version-less libcrypto|ssl.so,
which poses long-term maintenance problems. One could choose to link
straight with libcrypto.so.X.Y [or libcrypto.X.Y.so], but it would be
inconsistent with the way AIX [or Unix] does things.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6487)