buildroot

arch/mips: internal backend not suitable for some cores

Some cores are not supported by upstream gcc.

Use the newly-introduced symbol to state so, rather than have the
exclusion in the toolchain choice.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>