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://golang.org/cl/168270043