net/http: deflake BenchmarkServerFakeConnNoKeepAlive.

Fixes issue 5121.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7814046
cmd/cc, cmd/ld: do not overflow strings in symbol lookup.

R=golang-dev, dave, minux.ma
CC=golang-dev
https://codereview.appspot.com/7876044