kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if enabled

Architectures that are capable can select
HAVE_LD_DEAD_CODE_DATA_ELIMINATION to enable selection of that
option (as an EXPERT kernel option).
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/Kconfig

@@ -596,21 +596,6 @@ config CC_STACKPROTECTOR_AUTO
 
 endchoice
 
-config LD_DEAD_CODE_DATA_ELIMINATION
-	bool
-	help
-	  Select this if the architecture wants to do dead code and
-	  data elimination with the linker by compiling with
-	  -ffunction-sections -fdata-sections and linking with
-	  --gc-sections.
-
-	  This requires that the arch annotates or otherwise protects
-	  its external entry points from being discarded. Linker scripts
-	  must also merge .text.*, .data.*, and .bss.* correctly into
-	  output sections. Care must be taken not to pull in unrelated
-	  sections (e.g., '.text.init'). Typically '.' in section names
-	  is used to distinguish them from label names / C identifiers.
-
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help

init/Kconfig

@@ -1038,6 +1038,33 @@ config CC_OPTIMIZE_FOR_SIZE
 
 endchoice
 
+config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	bool
+	help
+	  This requires that the arch annotates or otherwise protects
+	  its external entry points from being discarded. Linker scripts
+	  must also merge .text.*, .data.*, and .bss.* correctly into
+	  output sections. Care must be taken not to pull in unrelated
+	  sections (e.g., '.text.init'). Typically '.' in section names
+	  is used to distinguish them from label names / C identifiers.
+
+config LD_DEAD_CODE_DATA_ELIMINATION
+	bool "Dead code and data elimination (EXPERIMENTAL)"
+	depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	depends on EXPERT
+	help
+	  Select this if the architecture wants to do dead code and
+	  data elimination with the linker by compiling with
+	  -ffunction-sections -fdata-sections, and linking with
+	  --gc-sections.
+
+	  This can reduce on disk and in-memory size of the kernel
+	  code and static data, particularly for small configs and
+	  on small systems. This has the possibility of introducing
+	  silently broken kernel if the required annotations are not
+	  present. This option is not well tested yet, so use at your
+	  own risk.
+
 config SYSCTL
 	bool