go.tools/cmd/benchcmp: do not generate trailing whitespace
go.tools/cmd/benchcmp: do not generate trailing whitespace

text/tabwriter cells are tab-terminated, not tab-separated. This creates trailing whitespace. However, with left-aligned columns, it is ok not to treat the final element as a cell. Demo: http://play.golang.org/p/m_ajG8SSZe

LGTM=gri
R=golang-codereviews, gobot, gri
CC=golang-codereviews, r
https://golang.org/cl/103650043

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