kernel: filter out compiler opts from config

kernel: filter out compiler opts from config

These get dynamically set based on compiler version. Not relevant for
targets.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16770
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit cd92cbddf8c473507a9af290c99f2aef8c368784)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>