x86-64: Fix unwind annotations in syscall stubs

With the return address removed from the stack, these should
really refer to their caller's register state.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
LKML-Reference: <4C7FBA3D0200007800013F61@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>