gcc-plugins: make it possible to disable CONFIG_GCC_PLUGINS again

I noticed that randconfig builds with gcc no longer produce a lot of
ccache hits, unlike with clang, and traced this back to plugins
now being enabled unconditionally if they are supported.

I am now working around this by adding

   export CCACHE_COMPILERCHECK=/usr/bin/size -A %compiler%

to my top-level Makefile. This changes the heuristic that ccache uses
to determine whether the plugins are the same after a 'make clean'.

However, it also seems that being able to just turn off the plugins is
generally useful, at least for build testing it adds noticeable overhead
but does not find a lot of bugs additional bugs, and may be easier for
ccache users than my workaround.

Fixes: 9f671e58 ("security: Create "kernel hardening" config area")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20191211133951.401933-1-arnd@arndb.de
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/Kconfig

@@ -14,8 +14,8 @@ config HAVE_GCC_PLUGINS
 	  An arch should select this symbol if it supports building with
 	  GCC plugins.
 
-config GCC_PLUGINS
-	bool
+menuconfig GCC_PLUGINS
+	bool "GCC plugins"
 	depends on HAVE_GCC_PLUGINS
 	depends on PLUGIN_HOSTCC != ""
 	default y
@@ -25,8 +25,7 @@ config GCC_PLUGINS
 
 	  See Documentation/core-api/gcc-plugins.rst for details.
 
-menu "GCC plugins"
-	depends on GCC_PLUGINS
+if GCC_PLUGINS
 
 config GCC_PLUGIN_CYC_COMPLEXITY
 	bool "Compute the cyclomatic complexity of a function" if EXPERT
@@ -113,4 +112,4 @@ config GCC_PLUGIN_ARM_SSP_PER_TASK
 	bool
 	depends on GCC_PLUGINS && ARM
 
-endmenu
+endif