gcc-plugins: Disable when building under Clang

Prior to doing compiler feature detection in Kconfig, attempts to build
GCC plugins with Clang would fail the build, much in the same way missing
GCC plugin headers would fail the build. However, now that this logic
has been lifted into Kconfig, add an explicit test for GCC (instead of
duplicating it in the feature-test script).
Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/gcc-plugins/Kconfig

@@ -2,7 +2,7 @@ preferred-plugin-hostcc := $(if-success,[ $(gcc-version) -ge 40800 ],$(HOSTCXX),
 
 config PLUGIN_HOSTCC
 	string
-	default "$(shell,$(srctree)/scripts/gcc-plugin.sh "$(preferred-plugin-hostcc)" "$(HOSTCXX)" "$(CC)")"
+	default "$(shell,$(srctree)/scripts/gcc-plugin.sh "$(preferred-plugin-hostcc)" "$(HOSTCXX)" "$(CC)")" if CC_IS_GCC
 	help
 	  Host compiler used to build GCC plugins.  This can be $(HOSTCXX),
 	  $(HOSTCC), or a null string if GCC plugin is unsupported.