go.tools/cmd/stringer: Fix build on 32bit OSs.
go.tools/cmd/stringer: Fix build on 32bit OSs.

LGTM=r
R=golang-codereviews, gri, r
CC=golang-codereviews
https://golang.org/cl/138040043

cw2018/tools,cmars/tools,matryer/tools,cmars/tools,golang/tools,josharian/tools,cmars/tools,sshetty10/tools,argandas/tools,sshetty10/tools,glasser/golang-tools,argandas/tools,golang/tools,golang/tools,josharian/tools,magiccao/tools,syreclabs/go-tools,magiccao/tools,magiccao/tools,4ad/tools,dmitris/tools,matryer/tools,jdhenke/tools,runcom/tools,springning/tools,matryer/tools,magiccao/tools,dmitris/tools,4ad/tools,golang/tools,glasser/golang-tools,cw2018/tools,cmars/tools,maddyonline/tools,nicot/tools,syreclabs/go-tools,golang/tools,matryer/tools,4ad/tools,dmitris/tools,sshetty10/tools,springning/tools,argandas/tools,josharian/tools,nicot/tools,cw2018/tools,maddyonline/tools,springning/tools,argandas/tools,dmitris/tools,syreclabs/go-tools,maddyonline/tools,josharian/tools,maddyonline/tools,jdhenke/tools,syreclabs/go-tools,jdhenke/tools,syreclabs/go-tools,nicot/tools,runcom/tools,glasser/golang-tools,cw2018/tools,jdhenke/tools,sshetty10/tools,runcom/tools,springning/tools,glasser/golang-tools,syreclabs/go-tools,runcom/tools,nicot/tools