mini-os/x86-64 entry: defer RESTORE_REST until return

mini-os/x86-64 entry: defer RESTORE_REST until return

No need to do a RESTORE_REST at this point because if we saw pending
events after we enabled event delivery, we have to do a SAVE_REST again.
Instead, we do a "lazy" RESTORE_REST, deferring it until actual return.
The offset of saved-on-stack rflags register is changed as well.

Signed-off-by: Xu Zhang <xzhang@cs.uic.edu>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>