kbuild: descend into scripts/gcc-plugins/ via scripts/Makefile

Now that 'archprepare' depends on 'scripts', Kbuild can descend into
scripts/gcc-plugins in a more standard way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
diff --git a/Makefile b/Makefile
index 06b6424..861d9d4 100644
--- a/Makefile
+++ b/Makefile
@@ -1110,7 +1110,7 @@
 
 macroprepare: prepare1 archmacros
 
-archprepare: archheaders archscripts macroprepare scripts gcc-plugins
+archprepare: archheaders archscripts macroprepare scripts
 
 prepare0: archprepare
 	$(Q)$(MAKE) $(build)=scripts/mod