package/gcc: fix download url when using upstream gcc for csky

package/gcc: fix download url when using upstream gcc for csky

When the csky cpu support was added [1], the gcc download url was selected
depending on the csky cpu architecture (BR2_csky) rather than the csky gcc
fork version (BR2_GCC_VERSION_CSKY)[2].

When adding gcc 9.x version [3], we forgot to update the condition in order
to use the url to the gcc csky fork only when BR2_GCC_VERSION_CSKY=y.

Due to this error, the toolchain build with the upstream gcc 9.x for csky
cpu is broken due a download error.

Fix this by using BR2_GCC_VERSION_CSKY instead of BR2_csky.

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/470072924

[1] 7873a5bd5ebbeb1674293dae6b06b50f0a1f2184
[2] https://git.buildroot.net/buildroot/tree/package/gcc/gcc.mk?id=7873a5bd5ebbeb1674293dae6b06b50f0a1f2184#n19
[3] 089000eccf584d611ad1a8e5400436952450e883

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>