configs/csky_gx6605s: bump gcc to 6.3.0, kernel to 4.9.56

configs/csky_gx6605s: bump gcc to 6.3.0, kernel to 4.9.56

This patch updates the csky custom external toolchain to fix a broken
host dependency of gdb on libexpat.so.0 (most hosts now ship
libexpat.so.1). gdb is required to copy and and boot the kernel (see
board/csky/readme.txt). The updated gdb no longer has this host
dependency, which solves the issue.

Since this external toolchain was built and tested with csky kernel
4.9.56, this patch also updates to that kernel version.

Finally, this 4.9.56 csky kernel changed its default bootargs. So we
add a patch to preserve the same behavior as the previous version.

Signed-off-by: Julien Olivain <juju@cotds.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>