cc/gcc: frontends are responsible for selecting the list of languages

Do for the final step the same as for the core step: compute the list
of selected langauages from the frontend, not in the backend.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Gustavo Zacarias <gustavo@zacarias.com.ar>"

 reporter_url="http://sourceware.org/ml/crossgcc/2011-03/msg00028.html"

 reporter_comment=""