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>

arch/Config.in

@@ -78,6 +78,9 @@ config BR2_aarch64_be
 config BR2_csky
 	bool "csky"
 	select BR2_ARCH_HAS_MMU_MANDATORY
+	# Most variants are supported by gcc-9+, except one that is
+	# handled as a special exception in package/gcc/Config.in.host
+	select BR2_ARCH_NEEDS_GCC_AT_LEAST_9
 	help
 	  csky is processor IP from china.
 	  http://www.c-sky.com/

arch/Config.in.csky

@@ -5,6 +5,8 @@ choice
 	  Specific CPU variant to use
 
 config BR2_ck610
+	# Not supported by upstream gcc <= 9, and handled as a special
+	# exception in package/gcc/Config.in.host
 	bool "ck610"
 
 config BR2_ck807

package/gcc/Config.in.host

@@ -89,6 +89,9 @@ config BR2_GCC_VERSION_9_X
 	# powerpc spe support has been deprecated since gcc 8.x.
 	# https://gcc.gnu.org/ml/gcc/2018-04/msg00102.html
 	depends on !BR2_powerpc_SPE
+	# C-SKY sk610 needs abiv1, which is not supported in
+	# upstream gcc
+	depends on !BR2_ck610
 	select BR2_TOOLCHAIN_GCC_AT_LEAST_9
 
 endchoice