runtime: fix missing error print in TestCgoSigfwd

The result of the call to fmt.Errorf was unused. It was clearly
intending to print the message, not simply construct an error.

Change-Id: I14856214c521a51fe4b45690e6c35fbb17e66577
Reviewed-on: https://go-review.googlesource.com/c/go/+/443375
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/runtime/testdata/testprogcgo/sigfwd.go

@@ -61,7 +61,7 @@ var nilPtr *byte
 func f() (ret bool) {
 	defer func() {
 		if recover() == nil {
-			fmt.Errorf("ERROR: couldn't raise SIGSEGV in Go.")
+			fmt.Fprintf(os.Stderr, "ERROR: couldn't raise SIGSEGV in Go\n")
 			C.exit(2)
 		}
 		ret = true