platform->sharedlib_simple(): return undef when same as platform->sharedlib()

platform->sharedlib_simple(): return undef when same as platform->sharedlib()

On some Unix-like platforms, there is no such thing as versioned shared
libraries.  In this case, platform->sharedlib_simple() should simply
return undef.  Among others, this avoids the shared libraries to be
installed as symlinks on themselves.

Fixes #16012

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16023)