cmd/gc: mark panicslice as unlikely

No measurable impact on performance on amd64

R=golang-dev, khr, bradfitz
CC=golang-dev
https://codereview.appspot.com/13096045

Committer: Brad Fitzpatrick <bradfitz@golang.org>
misc/emacs: allow users to customize path of 'go' tool.

Some users have multiple Go development trees and invoke the
'go' tool via a wrapper that sets GOROOT and GOPATH based on
the current directory.  Such users should customize go-command
to point to the wrapper script.

R=dominik.honnef
CC=golang-dev
https://codereview.appspot.com/13233043