cmd/compile/internal/gc: rename Fatal to Fatalf

This helps vet see a real issue:

    cmd/internal/gc$ go vet
    gen.go:1223: unreachable code

Fixes #12106.

Change-Id: I720868b07ae6b6d5a4dc6b238baa8c9c889da6d8
Reviewed-on: https://go-review.googlesource.com/14083
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/go.go b/src/cmd/compile/internal/gc/go.go
index be56b81..d31adb2 100644
--- a/src/cmd/compile/internal/gc/go.go
+++ b/src/cmd/compile/internal/gc/go.go
@@ -98,7 +98,7 @@
 func (v Val) Ctype() int {
 	switch x := v.U.(type) {
 	default:
-		Fatal("unexpected Ctype for %T", v.U)
+		Fatalf("unexpected Ctype for %T", v.U)
 		panic("not reached")
 	case nil:
 		return 0