Makefile: unconditionally disable SSP

Though -nostdlib is passed in CFLAGS, -fno-stack-protector must also be
passed to avoid linking errors related to undefined references to
'__stack_chk_guard' and '__stack_chk_fail' if toolchain enforces
-fstack-protector.

Fixes:
 - https://gitlab.com/kubu93/buildroot/-/jobs/1247043359



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>