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.