io: prevent write to PipeWriter after Close

Return an ErrClosedPipe rather than allowing the write to proceed.

Fixes issue 5330.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12541053

Committer: Brad Fitzpatrick <bradfitz@golang.org>
sync/atomic: fix ARM nomenclature in comments

R=cshapiro
CC=golang-dev
https://codereview.appspot.com/12877043
runtime: fix false deadlock crash
Fixes issue 6070.
Update issue 6055.

R=golang-dev, nightlyone, rsc
CC=golang-dev
https://codereview.appspot.com/12602043