MIPS: add support for interAptiv cores

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

Also disable external toolchains that don't support this core.

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