runtime: fix plan9 warning.
runtime: fix plan9 warning.

I have no idea what this code is for, but it pretty
clearly needs to be uint64, not uint32.

LGTM=aram
R=0intro, aram
CC=golang-codereviews
https://codereview.appspot.com/84410043

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