package/gcc: disable gcc 9.x for csky

package/gcc: disable gcc 9.x for csky

When building a toolchain with upstream gcc 9.x the build
fail due to several issues.

Note: The upstream Binutils support csky target since
release 2.32 but the support was never enabled in the
Buildroot packaging. So the latest version (2.33.1) was
tested here.

[upstream gcc 9.x w/ glibc csky fork with binutils csky for or binutils 2.33.1]
In file included from <command-line>:
./../include/libc-symbols.h:534:26: error: '__EI___errno_location' specifies less restrictive attributes than its target '__errno_location': 'const', 'nothrow' [-Werror=missing-attributes]
  534 |   extern __typeof (name) __EI_##name \

[upstream gcc 9.x w/ glibc 2.30 w/ binutils csky fork]
/tmp/ccThLRhb.s: Assembler messages:
/tmp/ccThLRhb.s:10: Error: invalid or unsupported encoding in .cfi_personality
/tmp/ccThLRhb.s:11: Error: invalid or unsupported encoding in .cfi_lsda

[upstream gcc 9.x w/ glibc 2.30 w/ binutils 2.33.1]
build/elf/librtld.os: in function `__sync_fetch_and_add_2':
libgcc/config/csky/linux-atomic.c:116: undefined reference to `__kernel_cmpxchg'

Currenlty, only the toolchain using binutils, gcc, glibc
fork produce a working toolchain. So disable gcc 9.x for
csky.

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