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>

# Alpha specific configuration file

config ARCH_alpha

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The Alpha architecture.