cmd/8c: disable use of prefetch with GO386=387
cmd/8c: disable use of prefetch with GO386=387

Fixes issue 4798.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7323061

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