kbuild: align W=e with CONFIG_WERROR

CONFIG_WERROR sets KBUILD_CPPFLAGS while W=e would only set KBUILD_CFLAGS.

As a preparation to unify the two mechanism, align their effects.

While at it, add some alignment whitespace to prepare for later additions
to the list of changed variables.

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-v2-1-c01e596309d2@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>