Use aux struct in libgomp for infrequently-used/API-specific data

	libgomp/
	* libgomp.h (struct splay_tree_aux): New.
	(struct splay_tree_key_s): Replace link_key field with aux pointer.
	* target.c (gomp_map_vars_internal): Adjust for link_key being moved
	to aux struct.
	(gomp_remove_var_internal): Free aux block if present.
	(gomp_load_image_to_device): Zero-initialise aux field instead of
	link_key field.
	(omp_target_associate_pointer): Zero-initialise aux field.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>

From-SVN: r279620