Fixes #5818: Correct the OpenSSL build on old OSes to use the right arch...

...itecture

Ticket: http://www.rudder-project.org/redmine/issues/5818

To be merged in 2.11
Ah, I just read the dpkg-architecture(1) man page:

>        build machine
>           The machine the package is built on.
>
>       host machine
>           The machine the package is built for.

So that's counter intuitive. But makes sense for this PR.