scripts: don't set LD_PRELOAD with static companion libraries

When cpmpanion libraries are built statically, it is useless to set
LD_PRELOAD. Worse, it might break the build in some corner cases.

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }