randstruct: Split randstruct Makefile and CFLAGS

To enable the new Clang randstruct implementation[1], move
randstruct into its own Makefile and split the CFLAGS from
GCC_PLUGINS_CFLAGS into RANDSTRUCT_CFLAGS.

[1] https://reviews.llvm.org/D121556

Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-5-keescook@chromium.org

scripts/Makefile.gcc-plugins

@@ -22,12 +22,6 @@ export DISABLE_STRUCTLEAK_PLUGIN
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)		\
 		+= -DSTRUCTLEAK_PLUGIN
 
-gcc-plugin-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)	+= randomize_layout_plugin.so
-gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)		\
-		+= -DRANDSTRUCT
-gcc-plugin-cflags-$(CONFIG_RANDSTRUCT_PERFORMANCE)		\
-		+= -fplugin-arg-randomize_layout_plugin-performance-mode
-
 gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK)	+= stackleak_plugin.so
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK)		\
 		+= -DSTACKLEAK_PLUGIN
@@ -60,6 +54,8 @@ KBUILD_CFLAGS += $(GCC_PLUGINS_CFLAGS)
 # be included in GCC_PLUGIN so they can get built.
 gcc-plugin-external-$(CONFIG_GCC_PLUGIN_SANCOV)			\
 	+= sancov_plugin.so
+gcc-plugin-external-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)		\
+	+= randomize_layout_plugin.so
 
 # All enabled GCC plugins are collected here for building in
 # scripts/gcc-scripts/Makefile.

scripts/Makefile.randstruct

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0
+
+randstruct-cflags-y += -DRANDSTRUCT
+
+ifdef CONFIG_GCC_PLUGIN_RANDSTRUCT
+randstruct-cflags-y	\
+	+= -fplugin=$(objtree)/scripts/gcc-plugins/randomize_layout_plugin.so
+randstruct-cflags-$(CONFIG_RANDSTRUCT_PERFORMANCE)		\
+	+= -fplugin-arg-randomize_layout_plugin-performance-mode
+endif
+
+export RANDSTRUCT_CFLAGS := $(randstruct-cflags-y)
+
+KBUILD_CFLAGS	+= $(RANDSTRUCT_CFLAGS)