[dev.cc] cmd/5g,cmd/6g,cmd/9g: fix warnings on Plan 9

warning: src/cmd/5g/reg.c:461 format mismatch d VLONG, arg 5
warning: src/cmd/6g/reg.c:396 format mismatch d VLONG, arg 5
warning: src/cmd/9g/reg.c:440 format mismatch d VLONG, arg 5

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/179300043