EVP: use evp_pkey_ctx_is_legacy() to find what implementation to use

We've had explicit checks for when to fall back to legacy code for
operations that use an EVP_PKEY.  Unfortunately, the checks were
radically different in different spots, so we refactor that into a
macro that gets used everywhere.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13043)