runtime: wipe out bitSpecial from GC code
runtime: wipe out bitSpecial from GC code

LGTM=khr, rsc
R=golang-codereviews, bradfitz, khr
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/72480044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang