scripts/build/internals.sh: Always use binary wrapper under BSD/MacOS

The shell wrapper script uses a nonportable call to readlink.
Thus, always use the binary wrapper under BSD/MacOS.

yann.morin.1998@anciens.enib.fr:
Use 'case' instead of 'if'.

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }