amd64 was done in CL 6275056.

We don't attempt to handle machines with clock speeds
less than 1 GHz. Those will fall back to the system call.

benchmark       old ns/op    new ns/op    delta
BenchmarkNow          364           38  -89.53%

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6307045