cc/gcc: pass the companion libs prefix to cc_core

In case of canadian-cross, the companion libraries are not the same for
the core cc (they run on 'build') as they are for the final cc (they run
on 'host').

Prepare for this differentiation (coming later), while retaining the
current behavior (to use the same compblibs).

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

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }