buildroot

arch/csky: internal backend not suitable

Upstream gcc does not have support for C-Sky, and we do not have a
vendor tree for it either (yet?).

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>