runtime: fix warnings on Plan 9
runtime: fix warnings on Plan 9

warning: pkg/runtime/mgc0.c:2352 format mismatch p UVLONG, arg 2
warning: pkg/runtime/mgc0.c:2352 format mismatch p UVLONG, arg 3

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/71950044

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