kbuild: unify W=e and CONFIG_WERROR

The two mechanisms have the same effect, unify their implementation.

Also avoid spurious rebuilds when switching between the two.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250814-kbuild-werror-v2-2-c01e596309d2@linutronix.de


Signed-off-by: Nathan Chancellor <nathan@kernel.org>