[nvptx] Remove use of CUDA unified memory in libgomp

[nvptx] Remove use of CUDA unified memory in libgomp

	libgomp/
	* plugin/plugin-nvptx.c (struct cuda_map): New.
	(struct ptx_stream): Replace d, h, h_begin, h_end, h_next, h_prev,
	h_tail with (cuda_map *) map.
	(cuda_map_create): New function.
	(cuda_map_destroy): New function.
	(map_init): Update to use a linked list of cuda_map objects.
	(map_fini): Likewise.
	(map_pop): Likewise.
	(map_push): Likewise.  Return CUdeviceptr instead of void.
	(init_streams_for_device): Remove stales references to ptx_stream
	members.
	(select_stream_for_async): Likewise.
	(nvptx_exec): Update call to map_init.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@264397 138bc75d-0d04-0410-961f-82ee72b054a4