kbuild: test dead code/data elimination support in Kconfig

This config option should be enabled only when both the compiler and
the linker support necessary flags.  Add proper dependencies to Kconfig.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -790,8 +790,8 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-fdata-sections,)
+KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
+LDFLAGS_vmlinux += --gc-sections
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
@@ -857,10 +857,6 @@ LDFLAGS_BUILD_ID := $(call ld-option, --build-id)
 KBUILD_LDFLAGS_MODULE += $(LDFLAGS_BUILD_ID)
 LDFLAGS_vmlinux += $(LDFLAGS_BUILD_ID)
 
-ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-LDFLAGS_vmlinux	+= $(call ld-option, --gc-sections,)
-endif
-
 ifeq ($(CONFIG_STRIP_ASM_SYMS),y)
 LDFLAGS_vmlinux	+= $(call ld-option, -X,)
 endif

init/Kconfig

@@ -1085,6 +1085,8 @@ config LD_DEAD_CODE_DATA_ELIMINATION
 	bool "Dead code and data elimination (EXPERIMENTAL)"
 	depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	depends on EXPERT
+	depends on $(cc-option,-ffunction-sections -fdata-sections)
+	depends on $(ld-option,--gc-sections)
 	help
 	  Enable this if you want to do dead code and data elimination with
 	  the linker by compiling with -ffunction-sections -fdata-sections,