libgomp: Simplify OpenMP reverse offload host <-> device memory copy implementation

libgomp: Simplify OpenMP reverse offload host <-> device memory copy implementation

... by using the existing 'goacc_asyncqueue' instead of re-coding parts of it.

Follow-up to commit 131d18e928a3ea1ab2d3bf61aa92d68a8a254609
"libgomp/nvptx: Prepare for reverse-offload callback handling",
and commit ea4b23d9c82d9be3b982c3519fe5e8e9d833a6a8
"libgomp: Handle OpenMP's reverse offloads".

	libgomp/
	* target.c (gomp_target_rev): Instead of 'dev_to_host_cpy',
	'host_to_dev_cpy', 'token', take a single 'goacc_asyncqueue'.
	* libgomp.h (gomp_target_rev): Adjust.
	* libgomp-plugin.c (GOMP_PLUGIN_target_rev): Adjust.
	* libgomp-plugin.h (GOMP_PLUGIN_target_rev): Adjust.
	* plugin/plugin-gcn.c (process_reverse_offload): Adjust.
	* plugin/plugin-nvptx.c (rev_off_dev_to_host_cpy)
	(rev_off_host_to_dev_cpy): Remove.
	(GOMP_OFFLOAD_run): Adjust.