runtime: show runtime.panic frame in traceback

Otherwise, if panic starts running deferred functions,
the code that panicked appears to be calling those
functions directly, which is not the case and can be
confusing.

For example:

main.Two()
        /Users/rsc/x.go:12 +0x2a
runtime.panic(0x20dc0, 0x2100cc010)
        /Users/rsc/g/go/src/pkg/runtime/panic.c:248 +0x106
main.One()
        /Users/rsc/x.go:8 +0x55

This makes clear(er) that main.Two is being called during
a panic, not as a direct call from main.One.

Fixes #5832.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13302051