1.0.9.50: O(1) weak pointer scavenging

1.0.9.50: O(1) weak pointer scavenging

* Use self-pointer instead of NULL to mark the end of scavenged weak
  pointer list in GC, which allows identifying unscavenged pointers
  by the NULL next pointers.

* Scavenging a single weak pointer in gencgc is now an O(1) operation
  instead of O(#scanned pointers so far).

Thanks to Paul Khuong.