test/bench/go1: add mandelbrot for floating point
test/bench/go1: add mandelbrot for floating point

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6244063
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go