Enable GC stats collection on demand

Today we require users to run their programs with `+RTS -T` or compile them with `-rtsopts` if they want GC stats. Turns out there's a way to enable GC stats during runtime:

http://comments.gmane.org/gmane.comp.lang.haskell.ghc.devel/5281

We should do this whenever the user requests GC stats to be monitored.
