On amd64, the arena is no longer in address space order, but currently
the heap dumper assumes that it is. Fix this assumption.

Change-Id: Iab1953cd36b359d0fb78ed49e5eb813116a18855
Reviewed-on: https://go-review.googlesource.com/96776
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>