[nvptx offloading] Only 64-bit configurations are currently supported

	PR libgomp/65099
	gcc/
	* config/nvptx/mkoffload.c (main): Create an offload image only in
	64-bit configurations.
	libgomp/
	* plugin/plugin-nvptx.c (nvptx_get_num_devices): Return 0 if not
	in a 64-bit configuration.
	* testsuite/libgomp.oacc-c++/c++.exp: Don't attempt nvidia
	offloading testing if no such device is available.
	* testsuite/libgomp.oacc-c/c.exp: Likewise.
	* testsuite/libgomp.oacc-fortran/fortran.exp: Likewise.

From-SVN: r225560