kernel: introduce KERNEL_WERROR config option

kernel: introduce KERNEL_WERROR config option

In commit b2d1eb717b65 ("generic: 5.15: enable Werror by default for
kernel compile") CONFIG_WERROR=y was enabled and all warnings/errors
reported with GCC 12 were fixed.

Keeping this in sync with past/future GCC versions is going to be uphill
battle, so lets introduce new KERNEL_WERROR config option, enable it by
default only for tested/known working combinations and on buildbots.

References: #12687
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit ce8c639a6c539534be14a540e7c3e9933d3a34ef)