[dev.regabi] add FatalfAt and fix Fatalf docs

I've wanted a FatalfAt function for a while, but under the old "-l"
suffix naming convention it would have been called "Fatalfl", which is
just atrocious.

Change-Id: If87f692ecdff478769426d4b054ac396e5c1e42e
Reviewed-on: https://go-review.googlesource.com/c/go/+/273013


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>