cc/gcc: add the backend/frontend infra for final gcc
Currently, we issue the bare-metal compiler from the pass_1 & pass_2
core compilers, because the final gcc breaks while doing so.
This implies we have to build some libces during the start_files step,
instead of the standard libc step. This is the case for newlib.
By adding a backend/frontend infra to the final gcc, we can abstract
what backend to call: the standard backend for non-bare-metal gcc,
and the core backend for bare-metal.
This patch is just an no-op, it just adds the final backend and
frontend without changing the way bare-metal is built, to come in a
subsequent patch.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>