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="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."