runtime: don't assume arena is in address order

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>

src/runtime/heapdump.go

@@ -491,10 +491,13 @@ func dumpparams() {
 	var arenaStart, arenaEnd uintptr
 	for i, ha := range mheap_.arenas {
 		if ha != nil {
-			if arenaStart == 0 {
-				arenaStart = arenaBase(uint(i))
+			base := arenaBase(uint(i))
+			if arenaStart == 0 || base < arenaStart {
+				arenaStart = base
+			}
+			if base+heapArenaBytes > arenaEnd {
+				arenaEnd = base + heapArenaBytes
 			}
-			arenaEnd = arenaBase(uint(i)) + heapArenaBytes
 		}
 	}
 	dumpint(uint64(arenaStart))