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
Well, the manpage of dpkg-architecture says:
```
       Doing something only for a specific architecture:

           DEB_HOST_ARCH ?= $(shell dpkg-architecture -qDEB_HOST_ARCH)

           ifeq ($(DEB_HOST_ARCH),alpha)
             [...]
           endif
```