arch/mips: add option for MIPS ABIs

This adds selection for one of the o32, n32 and n64 ABIs.
Later, we can easily use those boolean options, rather than
relying on a user-supplied string option.

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

}