test/bench/shootout: match gcc architecture to GOARCH
test/bench/shootout: match gcc architecture to GOARCH

If we're benchmarking 8g, use gcc -m32.
If we're benchmarking 6g, use gcc -m64.

R=golang-dev, bradfitz, minux.ma, remyoudompheng
CC=golang-dev
http://codereview.appspot.com/6625061

firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang