arch/powerpc: fix tuple for uClibc

Changeset #7c288c777455 broke the tuple for uClibc-based
powerpc toolchains, by unconditionally forcing CT_TARGET_SYS
to "gnu".

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

}