gcc-plugins: test plugin support in Kconfig and clean up Makefile

Run scripts/gcc-plugin.sh from Kconfig so that users can enable
GCC_PLUGINS only when the compiler supports building plugins.

Kconfig defines a new symbol, PLUGIN_HOSTCC.  This will contain
the compiler (g++ or gcc) used for building plugins, or empty
if the plugin can not be supported at all.

This allows us to remove all ugly testing in Makefile.gcc-plugins.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>

arch/Kconfig

@@ -398,6 +398,15 @@ config SECCOMP_FILTER
 
 	  See Documentation/prctl/seccomp_filter.txt for details.
 
+preferred-plugin-hostcc := $(if-success,[ $(gcc-version) -ge 40800 ],$(HOSTCXX),$(HOSTCC))
+
+config PLUGIN_HOSTCC
+	string
+	default "$(shell,$(srctree)/scripts/gcc-plugin.sh "$(preferred-plugin-hostcc)" "$(HOSTCXX)" "$(CC)")"
+	help
+	  Host compiler used to build GCC plugins.  This can be $(HOSTCXX),
+	  $(HOSTCC), or a null string if GCC plugin is unsupported.
+
 config HAVE_GCC_PLUGINS
 	bool
 	help
@@ -407,6 +416,7 @@ config HAVE_GCC_PLUGINS
 menuconfig GCC_PLUGINS
 	bool "GCC plugins"
 	depends on HAVE_GCC_PLUGINS
+	depends on PLUGIN_HOSTCC != ""
 	depends on !COMPILE_TEST
 	help
 	  GCC plugins are loadable modules that provide extra features to the