toolchain/buildroot: not available for a few mips cores

toolchain/buildroot: not available for a few mips cores

Commit 1b974425 (MIPS: add support for M6201 cores) explained that the
new core was not supported by upstream gcc, and as of gcc-8-trunk
that's still the case.

Ditto for 3cfbeb83 (MIPS: add support for P6600 cores).

This means that we currently allow to build an internal tolchain for
those cores, yet we have no suitable gcc version.

Disable the internal backend in this case.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>