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>