OpenACC 2.6 deep copy: attach/detach API routines

	libgomp/
	* libgomp.h (struct splay_tree_aux): Add attach_count field.
	(gomp_attach_pointer, gomp_detach_pointer): Add prototypes.
	* libgomp.map (OACC_2.6): New section. Add acc_attach,
	acc_attach_async, acc_detach, acc_detach_async, acc_detach_finalize,
	acc_detach_finalize_async.
	* oacc-mem.c (acc_attach_async, acc_attach, goacc_detach_internal,
	acc_detach, acc_detach_async, acc_detach_finalize,
	acc_detach_finalize_async): New functions.
	* openacc.h (acc_attach, acc_attach_async, acc_detach,
	(acc_detach_async, acc_detach_finalize, acc_detach_finalize_async): Add
	prototypes.
	* target.c (gomp_attach_pointer, gomp_detach_pointer): New functions.
	(gomp_remove_var_internal): Free attachment counts if present.
	* testsuite/libgomp.oacc-c-c++-common/deep-copy-3.c: New test.
	* testsuite/libgomp.oacc-c-c++-common/deep-copy-5.c: New test.

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

From-SVN: r279624