MIPS: add support for M5100 cores

-march=m5100 support starts from GCC-6, so disable previous versions
when selecting this core.

Note that M5100 implies a MIPS R5 CPU, and some GCC versions are already
disabled for R5, so we don't need to disable those ones for M5100 as
well.

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>