kbuild: consolidate warning flags in scripts/Makefile.extrawarn

Warning options are enabled and disabled in inconsistent ways and
inconsistent locations. Start rearranging those by moving all options
into Makefile.extrawarn.

This should not change any behavior, but makes sure we can group them
in a way that ensures that each warning that got temporarily disabled
is turned back on at an appropriate W=1 level later on.

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