libgomp: adjust nvptx_free callback context checking

libgomp: adjust nvptx_free callback context checking

Change test for CUDA callback context in nvptx_free() from using
GOMP_PLUGIN_acc_thread () into checking for CUDA_ERROR_NOT_PERMITTED,
for the former only works for OpenACC, but not OpenMP offloading.

2020-08-20  Chung-Lin Tang  <cltang@codesourcery.com>

	libgomp/
	* plugin/plugin-nvptx.c (nvptx_free):
	Change "GOMP_PLUGIN_acc_thread () == NULL" test into check of
	CUDA_ERROR_NOT_PERMITTED status for cuMemGetAddressRange. Adjust
	comments.