time: change float expression to ensure it is an integer
time: change float expression to ensure it is an integer

When I increased the number of bits that gccgo uses for
untyped floats, the expression 0.52*1e9 was no longer
integral.  This patch fixes that.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6113043

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