cmd/gc, cmd/ld: fix warnings on Plan 9

src/cmd/gc/closure.c:133 param declared and not used: nowrap
src/cmd/gc/const.c:1139 set and not used: t1
src/cmd/ld/data.c:652 format mismatch #llx INT, arg 7
src/cmd/ld/data.c:652 format mismatch #llx INT, arg 8
src/cmd/ld/data.c:1230 set and not used: datsize

R=dave, golang-dev, lucio.dere, remyoudompheng, bradfitz
CC=golang-dev
https://codereview.appspot.com/8182043

Committer: Brad Fitzpatrick <bradfitz@golang.org>