build: disable kernel stack protector support for i386/x86_64

When stack protector support is disabled in libc (always the case for
!musl), gcc assumes that it needs to use __stack_chk_guard for the stack
canary.
This causes kernel build errors, because the kernel is only set up to
handle TLS stack canaries.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 46543