runtime: don't stop bitmap dump at BitsDead

Stack bitmaps need to be scanned past any BitsDead entries.

Object bitmaps will not have any BitsDead in them (bitmap extraction stops at
the first BitsDead entry in makeheapobjbv).  data/bss bitmaps also have no BitsDead entries.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/168270043