cmd/dist, cmd/8g: implement GO386=387/sse to choose FPU flavour.

A new environment variable GO386 is introduced to choose between
code generation targeting 387 or SSE2. No auto-detection is
performed and the setting defaults to 387 to preserve previous
behaviour.

The patch is a reorganization of CL6549052 by rsc.

Fixes issue 3912.

R=minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/6962043