kbuild: add some extra warning flags unconditionally

These flags are documented in the GCC 4.6 manual, and recognized by
Clang as well. Let's rip off the cc-option / cc-disable-warning switches.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

scripts/Makefile.extrawarn

@@ -23,15 +23,16 @@ warning-  := $(empty)
 warning-1 := -Wextra -Wunused -Wno-unused-parameter
 warning-1 += -Wmissing-declarations
 warning-1 += -Wmissing-format-attribute
-warning-1 += $(call cc-option, -Wmissing-prototypes)
+warning-1 += -Wmissing-prototypes
 warning-1 += -Wold-style-definition
-warning-1 += $(call cc-option, -Wmissing-include-dirs)
+warning-1 += -Wmissing-include-dirs
 warning-1 += $(call cc-option, -Wunused-but-set-variable)
 warning-1 += $(call cc-option, -Wunused-const-variable)
 warning-1 += $(call cc-option, -Wpacked-not-aligned)
 warning-1 += $(call cc-option, -Wstringop-truncation)
-warning-1 += $(call cc-disable-warning, missing-field-initializers)
-warning-1 += $(call cc-disable-warning, sign-compare)
+# The following turn off the warnings enabled by -Wextra
+warning-1 += -Wno-missing-field-initializers
+warning-1 += -Wno-sign-compare
 
 warning-2 := -Waggregate-return
 warning-2 += -Wcast-align
@@ -39,8 +40,8 @@ warning-2 += -Wdisabled-optimization
 warning-2 += -Wnested-externs
 warning-2 += -Wshadow
 warning-2 += $(call cc-option, -Wlogical-op)
-warning-2 += $(call cc-option, -Wmissing-field-initializers)
-warning-2 += $(call cc-option, -Wsign-compare)
+warning-2 += -Wmissing-field-initializers
+warning-2 += -Wsign-compare
 warning-2 += $(call cc-option, -Wmaybe-uninitialized)
 warning-2 += $(call cc-option, -Wunused-macros)