R=golang-dev, bradfitz CC=golang-dev https://golang.org/cl/6827061
This change alters the assembly to use FSUBD instructions such that 6l will actually emit the correct FSUBRD instructions and enables the assembly code. R=golang-dev, r CC=golang-dev https://golang.org/cl/6514044
(Reference implementation by dchest. amd64 disabled pending 6l fix.) R=golang-dev, dchest CC=golang-dev https://golang.org/cl/6494105