Fixed Makefile so a null CUDA_ARCH is treated like an unset one. (#7515)
Fixed Makefile so a null CUDA_ARCH is treated like an unset one. (#7515)


tlby/mxnet,gautamkmr/incubator-mxnet,yajiedesign/mxnet,vikingMei/mxnet,jamesliu/mxnet,yuruofeifei/mxnet,TuSimple/mxnet,ptrendx/mxnet,precedenceguo/mxnet,Mega-DatA-Lab/mxnet,vikingMei/mxnet,vikingMei/mxnet,luoyetx/mxnet,ShownX/incubator-mxnet,sxjscience/mxnet,mbaijal/incubator-mxnet,TuSimple/mxnet,piiswrong/mxnet,yajiedesign/mxnet,antoan2/incubator-mxnet,mbaijal/incubator-mxnet,zhreshold/mxnet,Mega-DatA-Lab/mxnet,weleen/mxnet,Prasad9/incubator-mxnet,hpi-xnor/BMXNet,kevinthesun/mxnet,gautamkmr/incubator-mxnet,tlby/mxnet,sxjscience/mxnet,sxjscience/mxnet,jamesliu/mxnet,reminisce/mxnet,indhub/mxnet,reminisce/mxnet,LinkHS/incubator-mxnet,jiajiechen/mxnet,stefanhenneking/mxnet,hpi-xnor/BMXNet,ZihengJiang/mxnet,kkk669/mxnet,ShownX/incubator-mxnet,tornadomeet/mxnet,wangyum/mxnet,jiajiechen/mxnet,leezu/mxnet,hpi-xnor/BMXNet,navrasio/mxnet,tornadomeet/mxnet,fullfanta/mxnet,Prasad9/incubator-mxnet,vikingMei/mxnet,Mega-DatA-Lab/mxnet,smolix/incubator-mxnet,piiswrong/mxnet,tlby/mxnet,stefanhenneking/mxnet,solin319/incubator-mxnet,fullfanta/mxnet,CodingCat/mxnet,CodingCat/mxnet,rahul003/mxnet,jermainewang/mxnet,jermainewang/mxnet,LinkHS/incubator-mxnet,indhub/mxnet,solin319/incubator-mxnet,mbaijal/incubator-mxnet,zhreshold/mxnet,Mega-DatA-Lab/mxnet,solin319/incubator-mxnet,jermainewang/mxnet,mbaijal/incubator-mxnet,tornadomeet/mxnet,navrasio/mxnet,reminisce/mxnet,wangyum/mxnet,madjam/mxnet,madjam/mxnet,indhub/mxnet,zhreshold/mxnet,dmlc/mxnet,stefanhenneking/mxnet,solin319/incubator-mxnet,jamesliu/mxnet,jennyzhang0215/incubator-mxnet,jiajiechen/mxnet,szha/mxnet,fullfanta/mxnet,crazy-cat/incubator-mxnet,ptrendx/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,yuruofeifei/mxnet,gautamkmr/incubator-mxnet,tornadomeet/mxnet,stefanhenneking/mxnet,DickJC123/mxnet,ShownX/incubator-mxnet,indhub/mxnet,mbaijal/incubator-mxnet,precedenceguo/mxnet,LinkHS/incubator-mxnet,mbaijal/incubator-mxnet,jamesliu/mxnet,ForkedReposBak/mxnet,stefanhenneking/mxnet,wangyum/mxnet,ShownX/incubator-mxnet,reminisce/mxnet,yuruofeifei/mxnet,stefanhenneking/mxnet,kevinthesun/mxnet,hpi-xnor/BMXNet,eric-haibin-lin/mxnet,weleen/mxnet,antoan2/incubator-mxnet,ForkedReposBak/mxnet,precedenceguo/mxnet,Northrend/mxnet,kevinthesun/mxnet,Northrend/mxnet,TuSimple/mxnet,luoyetx/mxnet,szha/mxnet,CodingCat/mxnet,TuSimple/mxnet,antoan2/incubator-mxnet,ykim362/mxnet,sergeykolychev/mxnet,saurabh3949/mxnet,kkk669/mxnet,piiswrong/mxnet,yajiedesign/mxnet,luoyetx/mxnet,larroy/mxnet,Northrend/mxnet,reminisce/mxnet,tornadomeet/mxnet,DickJC123/mxnet,jiajiechen/mxnet,mbaijal/incubator-mxnet,Mega-DatA-Lab/mxnet,precedenceguo/mxnet,fullfanta/mxnet,solin319/incubator-mxnet,jiajiechen/mxnet,hpi-xnor/BMXNet,yuruofeifei/mxnet,ShownX/incubator-mxnet,apache/incubator-mxnet,solin319/incubator-mxnet,zhreshold/mxnet,ptrendx/mxnet,ykim362/mxnet,tlby/mxnet,tlby/mxnet,vikingMei/mxnet,wangyum/mxnet,madjam/mxnet,leezu/mxnet,ZihengJiang/mxnet,nicklhy/mxnet,jiajiechen/mxnet,dmlc/mxnet,sergeykolychev/mxnet,jamesliu/mxnet,tornadomeet/mxnet,saurabh3949/mxnet,weleen/mxnet,weleen/mxnet,saurabh3949/mxnet,leezu/mxnet,smolix/incubator-mxnet,ptrendx/mxnet,madjam/mxnet,navrasio/mxnet,zhreshold/mxnet,nicklhy/mxnet,hpi-xnor/BMXNet,ykim362/mxnet,tornadomeet/mxnet,smolix/incubator-mxnet,szha/mxnet,eric-haibin-lin/mxnet,leezu/mxnet,ForkedReposBak/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,TuSimple/mxnet,sxjscience/mxnet,ykim362/mxnet,leezu/mxnet,crazy-cat/incubator-mxnet,apache/incubator-mxnet,smolix/incubator-mxnet,DickJC123/mxnet,Prasad9/incubator-mxnet,ZihengJiang/mxnet,formath/mxnet,apache/incubator-mxnet,ShownX/incubator-mxnet,crazy-cat/incubator-mxnet,madjam/mxnet,ShownX/incubator-mxnet,fullfanta/mxnet,indhub/mxnet,CodingCat/mxnet,ykim362/mxnet,piiswrong/mxnet,rahul003/mxnet,sergeykolychev/mxnet,navrasio/mxnet,nicklhy/mxnet,zhreshold/mxnet,rahul003/mxnet,eric-haibin-lin/mxnet,rahul003/mxnet,larroy/mxnet,vikingMei/mxnet,precedenceguo/mxnet,tlby/mxnet,luoyetx/mxnet,ForkedReposBak/mxnet,antoan2/incubator-mxnet,crazy-cat/incubator-mxnet,larroy/mxnet,larroy/mxnet,jermainewang/mxnet,jiajiechen/mxnet,nicklhy/mxnet,yajiedesign/mxnet,crazy-cat/incubator-mxnet,zhreshold/mxnet,ptrendx/mxnet,Prasad9/incubator-mxnet,rahul003/mxnet,saurabh3949/mxnet,sergeykolychev/mxnet,tornadomeet/mxnet,Mega-DatA-Lab/mxnet,DickJC123/mxnet,kevinthesun/mxnet,madjam/mxnet,formath/mxnet,indhub/mxnet,solin319/incubator-mxnet,sxjscience/mxnet,Northrend/mxnet,kkk669/mxnet,kevinthesun/mxnet,reminisce/mxnet,CodingCat/mxnet,eric-haibin-lin/mxnet,jiajiechen/mxnet,vikingMei/mxnet,jamesliu/mxnet,jennyzhang0215/incubator-mxnet,larroy/mxnet,szha/mxnet,stefanhenneking/mxnet,rahul003/mxnet,navrasio/mxnet,sxjscience/mxnet,jermainewang/mxnet,smolix/incubator-mxnet,TuSimple/mxnet,fullfanta/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,kkk669/mxnet,vikingMei/mxnet,formath/mxnet,Northrend/mxnet,yajiedesign/mxnet,madjam/mxnet,LinkHS/incubator-mxnet,formath/mxnet,ykim362/mxnet,ShownX/incubator-mxnet,sergeykolychev/mxnet,hpi-xnor/BMXNet,ForkedReposBak/mxnet,Prasad9/incubator-mxnet,antoan2/incubator-mxnet,reminisce/mxnet,precedenceguo/mxnet,formath/mxnet,indhub/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,dmlc/mxnet,smolix/incubator-mxnet,luoyetx/mxnet,sxjscience/mxnet,jamesliu/mxnet,saurabh3949/mxnet,jermainewang/mxnet,szha/mxnet,rahul003/mxnet,nicklhy/mxnet,ykim362/mxnet,eric-haibin-lin/mxnet,yajiedesign/mxnet,dmlc/mxnet,ShownX/incubator-mxnet,jamesliu/mxnet,luoyetx/mxnet,LinkHS/incubator-mxnet,ptrendx/mxnet,kevinthesun/mxnet,saurabh3949/mxnet,rahul003/mxnet,navrasio/mxnet,jermainewang/mxnet,jennyzhang0215/incubator-mxnet,ptrendx/mxnet,ZihengJiang/mxnet,yajiedesign/mxnet,dmlc/mxnet,weleen/mxnet,piiswrong/mxnet,Mega-DatA-Lab/mxnet,jiajiechen/mxnet,jennyzhang0215/incubator-mxnet,precedenceguo/mxnet,szha/mxnet,madjam/mxnet,smolix/incubator-mxnet,Prasad9/incubator-mxnet,Prasad9/incubator-mxnet,larroy/mxnet,crazy-cat/incubator-mxnet,luoyetx/mxnet,ptrendx/mxnet,precedenceguo/mxnet,jermainewang/mxnet,Mega-DatA-Lab/mxnet,yuruofeifei/mxnet,wangyum/mxnet,crazy-cat/incubator-mxnet,LinkHS/incubator-mxnet,wangyum/mxnet,saurabh3949/mxnet,navrasio/mxnet,ZihengJiang/mxnet,larroy/mxnet,dmlc/mxnet,ZihengJiang/mxnet,yajiedesign/mxnet,vikingMei/mxnet,jermainewang/mxnet,LinkHS/incubator-mxnet,nicklhy/mxnet,mbaijal/incubator-mxnet,ZihengJiang/mxnet,sergeykolychev/mxnet,Prasad9/incubator-mxnet,zhreshold/mxnet,hpi-xnor/BMXNet,szha/mxnet,stefanhenneking/mxnet,tornadomeet/mxnet,jennyzhang0215/incubator-mxnet,ForkedReposBak/mxnet,yuruofeifei/mxnet,CodingCat/mxnet,nicklhy/mxnet,sergeykolychev/mxnet,navrasio/mxnet,formath/mxnet,leezu/mxnet,kevinthesun/mxnet,LinkHS/incubator-mxnet,leezu/mxnet,nicklhy/mxnet,kkk669/mxnet,ForkedReposBak/mxnet,szha/mxnet,madjam/mxnet,luoyetx/mxnet,saurabh3949/mxnet,jamesliu/mxnet,stefanhenneking/mxnet,yuruofeifei/mxnet,gautamkmr/incubator-mxnet,saurabh3949/mxnet,crazy-cat/incubator-mxnet,kkk669/mxnet,dmlc/mxnet,sergeykolychev/mxnet,smolix/incubator-mxnet,crazy-cat/incubator-mxnet,nicklhy/mxnet,reminisce/mxnet,LinkHS/incubator-mxnet,piiswrong/mxnet,yajiedesign/mxnet,TuSimple/mxnet,CodingCat/mxnet,tlby/mxnet,gautamkmr/incubator-mxnet,antoan2/incubator-mxnet,kevinthesun/mxnet,apache/incubator-mxnet,sxjscience/mxnet,kkk669/mxnet,wangyum/mxnet,weleen/mxnet,gautamkmr/incubator-mxnet,weleen/mxnet,wangyum/mxnet,sergeykolychev/mxnet,reminisce/mxnet,hpi-xnor/BMXNet,ptrendx/mxnet,weleen/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,ForkedReposBak/mxnet,navrasio/mxnet,CodingCat/mxnet,fullfanta/mxnet,fullfanta/mxnet,ZihengJiang/mxnet,sxjscience/mxnet,gautamkmr/incubator-mxnet,antoan2/incubator-mxnet,kkk669/mxnet,luoyetx/mxnet,larroy/mxnet,formath/mxnet,solin319/incubator-mxnet,zhreshold/mxnet,ptrendx/mxnet,piiswrong/mxnet,yuruofeifei/mxnet,Prasad9/incubator-mxnet,TuSimple/mxnet,piiswrong/mxnet,jennyzhang0215/incubator-mxnet,leezu/mxnet,rahul003/mxnet,kkk669/mxnet,CodingCat/mxnet,Mega-DatA-Lab/mxnet,gautamkmr/incubator-mxnet,gautamkmr/incubator-mxnet,leezu/mxnet,piiswrong/mxnet,solin319/incubator-mxnet,ykim362/mxnet,fullfanta/mxnet,kevinthesun/mxnet,smolix/incubator-mxnet,tlby/mxnet,formath/mxnet,tlby/mxnet,formath/mxnet,szha/mxnet,Northrend/mxnet,yuruofeifei/mxnet,larroy/mxnet,Northrend/mxnet,eric-haibin-lin/mxnet,Northrend/mxnet,eric-haibin-lin/mxnet,dmlc/mxnet,antoan2/incubator-mxnet,weleen/mxnet,indhub/mxnet,antoan2/incubator-mxnet,Northrend/mxnet,reminisce/mxnet,ykim362/mxnet,jennyzhang0215/incubator-mxnet,ZihengJiang/mxnet,indhub/mxnet,precedenceguo/mxnet,wangyum/mxnet