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: nodejs/node-v0.x-archive#9451
Reviewed-By: Julien Gilli <julien.gilli@joyent.com>

PR: #25523
PR-URL: nodejs/node-v0.x-archive#25523
Reviewed-By: Julien Gilli <jgilli@fastmail.fm>

PR: #25654
PR-URL: nodejs/node-v0.x-archive#25654
Reviewed-By: Julien Gilli <jgilli@fastmail.fm>

Author: Shigeki Ohtsu