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 IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}