nvptx offloading: Global constructor, destructor support, via nvptx-tools 'ld'

This extends commit d9c90c82
"nvptx target: Global constructor, destructor support, via nvptx-tools 'ld'"
for offloading.

	libgcc/
	* config/nvptx/gbl-ctors.c ["mgomp"]
	(__do_global_ctors__entry__mgomp)
	(__do_global_dtors__entry__mgomp): New.
	[!"mgomp"] (__do_global_ctors__entry, __do_global_dtors__entry):
	New.
	libgomp/
	* plugin/plugin-nvptx.c (nvptx_do_global_cdtors): New.
	(nvptx_close_device, GOMP_OFFLOAD_load_image)
	(GOMP_OFFLOAD_unload_image): Call it.