debug/cmd/viewcore: fix printing of offsets

If the destination of a pointer is past the known typing,
just print the offset directly.

Change-Id: I1794d53065d2be4e74df65d50a803e97ea1a34fe
Reviewed-on: https://go-review.googlesource.com/102735
Reviewed-by: Peter Weinberger <pjw@google.com>