Update #7347

When runtime.panic is called the *Panic is malloced from the heap. This can lead to a gc cycle while panicing which can make a bad situation worse.

It appears to be possible to stack allocate the Panic and avoid malloc'ing during a panic.

Ref: https://groups.google.com/d/topic/golang-dev/OfxqpklGkh0/discussion

LGTM=minux.ma, dvyukov, rsc
R=r, minux.ma, gobot, rsc, dvyukov
CC=golang-codereviews
https://golang.org/cl/66830043