re PR go/64573 (Cmd.Start swallows error from fork()/clone())

	PR go/64573
syscall: Restore line somehow lost in libgo merge.

From-SVN: r220068

libgo/go/syscall/exec_unix.go

@@ -226,6 +226,7 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 	// Kick off child.
 	pid, err1 = forkAndExecInChild(argv0p, argvp, envvp, chroot, dir, attr, sys, p[1])
 	if err1 != 0 {
+		err = Errno(err1)
 		goto error
 	}
 	ForkLock.Unlock()