runtime: fix float64 hash on 32-bit machine
runtime: fix float64 hash on 32-bit machine

Multiplying by the low 32 bits was a bad idea
no matter what, but it was a particularly unfortunate
choice because those bits are 0 for small integer values.

Fixes issue 2883.

R=ken2
CC=golang-dev
http://codereview.appspot.com/5634047

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang