runtime: if span was scavenged, mark it as needing to be zeroed

Update issue 4979.

R=dvyukov, r, bradfitz
CC=golang-dev
https://codereview.appspot.com/8697045
cmd/dist, make.bash: pass -mmacosx-version-min=10.6 to gcc on Darwin
Fixes issue 5261.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8798044

Committer: Rob Pike <r@golang.org>
doc/effective_go.html: fix typo
Fixes issue 5296.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8682044
doc/effective_go: another doubled word
It's clear that that should never happen.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8798045
doc: more doubled words
I wrote a tool.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8695047