Per architecture restorer trampolines

Some architectures like ppc64 requires a trampoline to be called prior to
the standard restorer services.

This patch introduces 3 trampolines which can be overwritten by
architectures in arch/x/include/asm/restore.h:
 - arch_export_restore_thread
 - arch_export_restore_task
 - arch_export_unmap

The architecture which doesn't need to overwrite them, has nothing to do.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>