arch/csky: restrict ck610 to the C-SKY gcc port

As Guo explained, upstream gcc does not support abi-v1 (only abi-v2), but
ck610 needs abi-v1 [0] [1]

To simplify things, we make the whole C-SKY architecture require gcc-9
or later, and add a single exception in gcc to force the ck610 to use
the C-SKY port.

Note that this does not change the default gcc version to be used for
C-SKY: the C-SKY port is still always the default one; the gcc-9 version
is only proposed as an alternative (except for ck610, of course).

[0] http://lists.busybox.net/pipermail/buildroot/2019-July/254386.html


[1] package/Makefile.in#73

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Guo Ren <guoren@kernel.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@gmail.com>
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>