cc/gcc: pass the install prefix to the core passes

Currently, the discrimination on the core compilers prefixes depends on
the type of core compiler to build.

This is not correct, and the caller of the core backend should specify
the prefix.

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

 }