kbuild: enable extra warnings by default

x86 allmodconfig builds without additional warnings after doing
this, so enable -Wextra (minus -Wmissing-field-initializers,
-Wsign-compare and -Wtype-limits), as well as -Wold-style-definition
and -Wmissing-include-dirs now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>