Updates to FFI spec: performGC

[Alastair Reid]

I think the thing to do is add the existing performGC to the standard
(perhaps giving it an hs_ prefix in the process) and leave development
of an extended version of the function for when the GHC folk (or
anyone else with a generational collector) decide they want a
forcefulness argument.  Come that day, we'd define:

  void performGC(void) { performPartialGC(0); }

(or whatever it is you do to force a full collection).

A