strings: speed up byteReplacer.Replace

benchmark                         old ns/op    new ns/op    delta
BenchmarkByteReplacerWriteString       7359         3661  -50.25%

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/102550043
runtime/race: update runtime to tip
This requires minimal changes to the runtime hooks. In particular,
synchronization events must be done only on valid addresses now,
so I've added the additional checks to race.c.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/101000046
runtime/race: update linux runtime
It was built on an old, bogus revision.

LGTM=minux
TBR=iant
R=iant, minux
CC=golang-codereviews
https://codereview.appspot.com/101370052
runtime/race: support freebsd
All tests pass except one test in regexp package.

LGTM=iant
R=golang-codereviews, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/107270043
run.bash: run race tests on freebsd

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/102580043
doc: say that race detector is supported on freebsd

LGTM=ruiu
R=golang-codereviews, ruiu
CC=golang-codereviews
https://codereview.appspot.com/109100046
doc: say that race detector is supported on freebsd

R=golang-codereviews
CC=golang-codereviews
https://codereview.appspot.com/103520044
strings: use sync.Pool to cache buffer

benchmark                         old ns/op    new ns/op    delta
BenchmarkByteReplacerWriteString       3596         3094  -13.96%

benchmark                        old allocs   new allocs    delta
BenchmarkByteReplacerWriteString          1            0  -100.00%

LGTM=dvyukov
R=bradfitz, dave, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/101330053
test: add test case for issue 8074.

Fixes issue 8074.

The issue was not reproduceable by revision

go version devel +e0ad7e329637 Thu Jun 19 22:19:56 2014 -0700 linux/arm

But include the original test case in case the issue reopens itself.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/107290043