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>

 # Compute MIPS-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple, override only for 64-bit

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="mips64${target_endian_el}"

     else

         # The architecture part of the tuple:

         CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

     fi

     # Override CFLAGS for endianness:

     case "${CT_ARCH_ENDIAN}" in

         big)    CT_ARCH_ENDIAN_CFLAG="-EB";;

         little) CT_ARCH_ENDIAN_CFLAG="-EL";;

     esac

     # Override ABI flags

     CT_ARCH_ABI_CFLAG="-mabi=${CT_ARCH_mips_ABI}"

     CT_ARCH_WITH_ABI="--with-abi=${CT_ARCH_mips_ABI}"

 }