cmd/compile: make fmt available in rewrite rules

cmd/compile: make fmt available in rewrite rules

During development and debugging, I often want to
write noteRule(fmt.Sprintf(...)), and end up
manually adding the import to the generated code.
Let's just make it always available instead.

Change-Id: I1e2d47c98ba056e1b5da42e35fb6ad26f1d9cc3d
Reviewed-on: https://go-review.googlesource.com/c/145207
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>