x/debug: handle page size differences

The page size of the core dump writer and reader may differ.
Handle that fact.

Change-Id: I81bb7fc946c1aed6f8df71056aae24668a7293de
Reviewed-on: https://go-review.googlesource.com/88156
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>