MIPS: add support for I6400 cores

-march=i6400 support starts from GCC-6, so disable previous versions
when selecting this core.

Note that I6400 implies a MIPS R6 CPU, and some GCC versions are already
disabled for R6, so we don't need to disable those ones for I6400 as
well.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>