Fix EVP_PKEY_eq() to be possible to use with strictly private keys

Fix EVP_PKEY_eq() to be possible to use with strictly private keys

EVP_PKEY_eq() assumed that an EVP_PKEY always has the public key
component if it has a private key component.  However, this assumption
no longer strictly holds true, at least for provider backed keys.
EVP_PKEY_eq() therefore needs to be modified to specify that the
private key should be checked too (at the discretion of what's
reasonable for the implementation doing the actual comparison).

Fixes #16267

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16765)