Fix speed sm2 bug

Should create PKEY CTX with EVP_PKEY_SM2;
each job should have its own sm2_pkey;
loopargs[i].sigsize should be set after EVP_DigestSign().

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14144)
