- Implement a small GC optimisation: when a static constructor has
  been determined to have no (indirect) CAF references, we set its
  static link field to a non-zero value (currently 1).  This prevents
  the garbage collector from traversing this closure and transitively
  everything it points to, and thus should speed up GC a little.

- Omit the static link field from static constructors which have no
  pointer fields (i.e. they are CONSTR_NOCAF_STATIC).

- Add the padding words and the static link field for a static
  constructor at (AbsC) code generation time, rather than in the back
  ends.  This eliminates some duplication between PprAbsC and
  AbsCStixGen.