Add long links support to Boehm.

Add long links support to Boehm.

	* gc.h: Added parts of boehm 7.3a long link API.

	* finalize.c (GC_finalize): Implemented long link support by adapting
	code from boehm 7.3a.
	The idea is to have two table of links, one for regular and another
	for the long links. Regular links are processed before finalization
	and the long ones right after. The bulk of the changes are abstracting
	the disappearing link hashtable into a data structure so we can have
	two of them.

	Boehm code came from github.com/ivmai/bdwgc.git. Authors of the
	relevant changes are Zach Saw and Ivan Maidanski.