x86: restore vsyscall64 prochandler

a recent fix:

  commit ce28b986


  Author: Thomas Gleixner <tglx@linutronix.de>
  Date:   Wed Feb 20 23:57:30 2008 +0100

    x86: fix vsyscall wreckage

removed the broken /kernel/vsyscall64 handler completely.
This triggers the following debug check:

  sysctl table check failed: /kernel/vsyscall64  No proc_handler

Restore the sane part of the proc handler.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>