The code for call site-specific pointer bitmaps was not ready in time,
but the zeroing required without it is too expensive to use by default.
We will have to wait for precise collection of stack frames until Go 1.3.

The precise collection can be re-enabled by

        GOEXPERIMENT=precisestack ./all.bash

but that will not be the default for a Go 1.2 build.

Fixes #6087.

R=golang-dev, jeremyjackins, dan.kortschak, r
CC=golang-dev
https://golang.org/cl/13677045