kbuild: avoid duplicate warning options

Some warning options are disabled at one place and then conditionally
re-enabled later in scripts/Makefile.extrawarn.

For consistency, rework this file so each of those warnings only
gets etiher enabled or disabled based on the W= flags but not both.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>