monitor: Check whether TCG is enabled before running the "info jit" code

The "info jit" command currently aborts on Mac OS X with the message
"qemu_mutex_lock: Invalid argument" when running with "-M accel=qtest".
We should only call into the TCG code here if TCG has really been
enabled and initialized.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1493179907-22516-1-git-send-email-thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
