kbuild: LD_DEAD_CODE_DATA_ELIMINATION no -ffunction-sections/-fdata-sections for module build

Modules do not tend to cope with -ffunction-sections, even though they
do not link with -gc-sections. It may be possible for unused symbols to
be trimmed from modules, but in general that would take much more work
in architecture module linker scripts.

For now, enable these only for kernel build.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -799,8 +799,8 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-ffunction-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-fdata-sections,)
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included