nvptx: 'cuDeviceGetCount' failure is fatal

Per commit 683f1184
"OpenMP: Move omp requires checks to libgomp", we're now using 'return -1'
from 'GOMP_OFFLOAD_get_num_devices' for 'omp_requires_mask' purposes.  This
missed that via 'nvptx_get_num_devices', we could also 'return -1' for
'cuDeviceGetCount' failure.  Before, this meant (in 'gomp_target_init') to
silently ignore the plugin/device -- which also has been doubtful behavior.
Let's instead turn 'cuDeviceGetCount' failure into a fatal error, similar to
other errors during device initialization.

	libgomp/
	* plugin/plugin-nvptx.c (nvptx_get_num_devices):
	'cuDeviceGetCount' failure is fatal.