deps: separate sha256/sha512-x86_64.pl for openssl

sha256-x86_64.pl does not exist in the origin openssl distribution. It
was copied from sha512-x86_64.pl and both sha256/sha512 scripts were
modified so as to generates only one asm file specified as its key
hash length.

PR: #9451
PR-URL: https://github.com/joyent/node/pull/9451


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>