x86, vdso: teach 'make clean' remove vdso64 binaries

After 'make clean' vdso64.so and vdso64.dbg.so were left in arch/x86/vdso/.

Link: http://lkml.kernel.org/r/1422453867-17326-1-git-send-email-andrej.skvortzov@gmail.com
Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>

arch/x86/vdso/Makefile

@@ -205,4 +205,4 @@ $(vdso_img_insttargets): install_%: $(obj)/%.dbg $(MODLIB)/vdso FORCE
 PHONY += vdso_install $(vdso_img_insttargets)
 vdso_install: $(vdso_img_insttargets) FORCE
 
-clean-files := vdso32-syscall* vdso32-sysenter* vdso32-int80*
+clean-files := vdso32-syscall* vdso32-sysenter* vdso32-int80* vdso64*