build: disable precise collection of stack frames

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 issue 6087.

R=golang-dev, jeremyjackins, dan.kortschak, r
CC=golang-dev
https://codereview.appspot.com/13677045
cmd/nm: make -S listing a bit more accurate

Hide container symbols like text and etext so that
the individual pieces inside are shown instead.
For example, if text and main.init have the same
address, it was a toss-up which name was printed.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13722046