net/http/httptest: fix incorrectly-named ResponseRecorder example

R=golang-dev, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/8545047
fmt: document that if String or Error panics, that value is printed

Fixes issue 5350.

R=r, minux.ma
CC=golang-dev
https://codereview.appspot.com/8950043
runtime: fix stack pointer corruption in runtime.cgocallback_gofunc()
runtime.setmg() calls another function (cgo_save_gm), so it must save
LR onto stack.
Re-enabled TestCthread test in misc/cgo/test.

Fixes issue 4863.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9019043
fmt: fix crash for Printf("%.", 3)
Fixes issue 5311

R=golang-dev, bradfitz, iant
CC=golang-dev
https://codereview.appspot.com/8961050
encoding/json: document that marshaling invalid utf-8 sequence will return error
Also added docs for InvalidUTF8Error.
Fixes issue 5360.

R=golang-dev, adg, r
CC=golang-dev
https://codereview.appspot.com/8926046
cmd/ld: fix some 64-bit issues
A few places in the linker pushed 64-bit values through 32-bit holes,
including in relocation.
Clean them up, and check for a few other overflows as well.
Tests to follow.

R=dsymonds
CC=gobot, golang-dev
https://codereview.appspot.com/9032043
cmd/gc: fix some overflows in the compiler
Some 64-bit fields were run through 32-bit words, some counts were
not checked for overflow, and relocations must fit in 32 bits.
Tests to follow.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/9033043
cmd/ld: disable relocation range check so build can go green while we debug the issue.

R=golang-dev, dsymonds, dave
CC=golang-dev
https://codereview.appspot.com/9038043