crypto: make SignBase compatible with OpenSSL 1.1.0

1.1.0 requires EVP_MD_CTX be heap-allocated. In doing so, move the Init
and Update hooks to shared code because they are the same between Verify
and Sign.

PR-URL: https://github.com/nodejs/node/pull/16130
Backport-PR-URL: https://github.com/nodejs/node/pull/18622


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>