nvptx: libgomp+mkoffload.cc: Prepare for reverse offload fn lookup

nvptx: libgomp+mkoffload.cc: Prepare for reverse offload fn lookup

Add support to nvptx for reverse lookup of function name to prepare for
'omp target device(ancestor:1)'.

gcc/ChangeLog:

	* config/nvptx/mkoffload.cc (struct id_map): Add 'dim' member.
	(record_id): Store func name without quotes, store dim separately.
	(process): For GOMP_REQUIRES_REVERSE_OFFLOAD, check that -march is
	at least sm_35, create '$offload_func_table' global array and init
	with reverse-offload function addresses.
	* config/nvptx/nvptx.cc (write_fn_proto_1, write_fn_proto): New
	force_public attribute to force .visible.
	(nvptx_declare_function_name): For "omp target
	device_ancestor_nohost" attribut, force .visible/TREE_PUBLIC.

libgomp/ChangeLog:

	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_load_image): Read offload
	function address table '$offload_func_table' if rev_fn_table
	is not NULL.