runtime: convert panic/recover to Go

created panic1.go just so diffs were available.
After this CL is in, I'd like to move panic.go -> defer.go
and panic1.go -> panic.go.

LGTM=rsc
R=rsc, khr
CC=golang-codereviews
https://codereview.appspot.com/133530045

Committer: Russ Cox <rsc@golang.org>