cmd/cgo: print the right error if mangling detection gccgo fails

Change-Id: I2324f6f51d2bf8a4ae1b139b3933bc78dfa75835
Reviewed-on: https://go-review.googlesource.com/c/158718
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/cgo/out.go

@@ -1255,7 +1255,7 @@ func determineGccgoManglingScheme() bool {
 	cmd := exec.Command(gccgocmd, "-S", "-o", "-", gofilename)
 	buf, cerr := cmd.CombinedOutput()
 	if cerr != nil {
-		fatalf("%s", err)
+		fatalf("%s", cerr)
 	}
 
 	// New mangling: expect go.l..u00e4ufer.Run