Use 'GOMP_MAP_VARS_TARGET' for OpenACC compute constructs [PR90596]

Use 'GOMP_MAP_VARS_TARGET' for OpenACC compute constructs [PR90596]

Thereby considerably simplify the device plugins' 'GOMP_OFFLOAD_openacc_exec',
'GOMP_OFFLOAD_openacc_async_exec' functions: in terms of lines of code, but in
particular conceptually: no more device memory allocation, host to device data
copying, device memory deallocation -- 'GOMP_MAP_VARS_TARGET' does all that for
us.

This depends on commit 2b2340e236c0bba8aaca358ea25a5accd8249fbd
"Allow libgomp 'cbuf' buffering with OpenACC 'async' for 'ephemeral' data",
where I said that "a use will emerge later", which is this one here.

	PR libgomp/90596
	libgomp/
	* target.c (gomp_map_vars_internal): Allow for
	'param_kind == GOMP_MAP_VARS_OPENACC | GOMP_MAP_VARS_TARGET'.
	* oacc-parallel.c (GOACC_parallel_keyed): Pass
	'GOMP_MAP_VARS_TARGET' to 'goacc_map_vars'.
	* plugin/plugin-gcn.c (alloc_by_agent, gcn_exec)
	(GOMP_OFFLOAD_openacc_exec, GOMP_OFFLOAD_openacc_async_exec):
	Adjust, simplify.
	(gomp_offload_free): Remove.
	* plugin/plugin-nvptx.c (nvptx_exec, GOMP_OFFLOAD_openacc_exec)
	(GOMP_OFFLOAD_openacc_async_exec): Adjust, simplify.
	(cuda_free_argmem): Remove.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-parallel-1.c:
	Adjust.