Add GHC.Stats.gcStatsEnabled

[tibbe]

It would be nice if GHC.Stats exported a new, top-level constant, gcStatsEnabled :: Bool, that's set to True if the program was run with RTS -T or equivalent. In addition we could consider throwing an exception if getGCStats was called when gcStatsEnabled = False. This would let programs check if GC stats collection is enabled and still do something reasonable if they're not. This would be useful in the ekg package.

Open question: could the value of +RTS -T change at runtime? If so, perhaps we instead want getGCStatsEnabled :: IO Bool.