build/internals.sh: Replace nonportable call to stat

The call to stat to find out if a file is a symlink works only on GNU systems,
and the replacing portable call to readlink is also shorter and more concise code.

yann.morin.1998@anciens.enib.fr:
Apply simpler test, after discussion with author and Arnaud LACOMBE on the ML.

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}