labelThread should work with just eventlog enabled RTS

It is unclear to myself and the one and only @bgamari why labelThead needs to also use the -debug RTS. It would useful to use with the normal eventlog to label threads. In my mind it is similar to something like traceEvent and traceMarker which do not require the debug RTS to work.

Someone should investigate why this is, sounds like a good task for a newcomer.