runtime: ensure forward progress when unwinding an arm stack frame

The arm gentraceback mishandled frame linkage values pointing
to the assembly return function.  This function is special as
its frame size is zero and it contains only one instruction.
These conditions would preserve the frame pointer and result
in an off by one error when unwinding the caller.

Fixes issue 5124

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8023043
doc/go1.1.html: net/mail, net/smtp

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7835049
reflect: add garbage collection info in ChanOf, MapOf, PtrTo, SliceOf

ArrayOf will remain unexported (and therefore unused) for Go 1.1.

Fixes issue 4375.

R=0xe2.0x9a.0x9b, r, iant
CC=golang-dev
https://codereview.appspot.com/7716048

Committer: Rob Pike <r@golang.org>