config: bump linux kernel to 4.8.6 in synopsys defconfigs

config: bump linux kernel to 4.8.6 in synopsys defconfigs

With this commit we update ARC defconfigs with the following:

  - "snps_axs101_defconfig", "snps_axs103_defconfig" and
    "snps_hs38_smp_vdk_defconfig":
     - bump linux kernel version to 4.8.6
     - set up host linux headers to 4.8

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
[Thomas: as reported by Alexey, this commit is in fact a bug fix: it
makes the defconfig work properly. Indeed, since the ARC compiler has
moved to gcc 6.x, the ABI has changed, and a Linux 4.8+ kernel is
needed for userspace to work on ARC.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>