Fix potential race condition in OpenACC "exit data" operations

	PR libgomp/92881

	libgomp/
	* libgomp.h (gomp_remove_var_async): Add prototype.
	* oacc-mem.c (delete_copyout): Call gomp_remove_var_async instead of
	gomp_remove_var.
	* target.c (gomp_unref_tgt): Change return type to bool, indicating
	whether target_mem_desc was unmapped.
	(gomp_unref_tgt_void): New.
	(gomp_remove_var): Reimplement in terms of...
	(gomp_remove_var_internal): ...this new helper function.
	(gomp_remove_var_async): New, implemented using above helper function.
	(gomp_unmap_vars_internal): Use gomp_unref_tgt_void instead of
	gomp_unref_tgt.

Reviewed-by: Thomas Schwinge <thomas@codesourcery.com>

From-SVN: r279388