runtime: precise garbage collection of channels

This changeset adds a mostly-precise garbage collection of channels.
The garbage collection support code in the linker isn't recognizing
channel types yet.

Fixes issue http://stackoverflow.com/questions/14712586/memory-consumption-skyrocket

R=dvyukov, rsc, bradfitz
CC=dave, golang-dev, minux.ma, remyoudompheng
https://codereview.appspot.com/7307086

Committer: Russ Cox <rsc@golang.org>