support/gnuconfig/config.sub: add C-SKY support

Modify config.sub so that it knows about the C-SKY
architecture. Without this, all autotools projects fail to build on
C-SKY.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
[Thomas: improved commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>