docs/known-issues: add the infamous "mixed implicit and normal rules"

make-3.82 now refuses to have both normal and implicit rules in the
same definition, so document the issue with a known workaround.

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

}