x86: delete vsyscall files during make clean

make clean failed to delete a few files in
x86/kernel. This is because kbuild does not
see the correct/full kernel/Makefile.

As a workaround until the Makefiles are merged specify
the files to be deleted in the common Makefile.

Reported by Mike Galbraith <efault@gmx.de>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>