EVP: Add a temporary SM2 hack to key generation

EVP: Add a temporary SM2 hack to key generation

The reason to do this is many-fold.  We need EC key generation for
other work.  However, SM2 are currently closely related to EC keys
with legacy methods, but not with provider methods.

To avoid having to wait on provider support for SM2, we temporarly
do an extra check for what the legacy methods identify as SM2 keys
(either the EVP_PKEY_SM2 pkey id was used, or the SM2 curve), and
redirect to legacy code in one case, and in the other case, we
forcedly downgrade provider side EC keys with SM2 curves to legacy
SM2 keys, using available tools.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11328)