[libgomp, nvptx] Allow cuGetErrorString to be NULL

[libgomp, nvptx] Allow cuGetErrorString to be NULL

Cuda driver api function cuGetErrorString is available in version 6.0 and
higher.

Currently, when the driver that is used does not contain this function, the
libgomp nvptx plugin will not build (PLUGIN_NVPTX_DYNAMIC == 0) or run
(PLUGIN_NVPTX_DYNAMIC == 1).

This patch fixes this problem by testing for the presence of the function, and
handling absence.

Build on x86_64 with nvptx accelerator and reg-tested libgomp, both with and
without --without-cuda-driver.

2018-08-08  Tom de Vries  <tdevries@suse.de>

	* plugin/cuda-lib.def (cuGetErrorString): Use CUDA_ONE_CALL_MAYBE_NULL.
	* plugin/plugin-nvptx.c (cuda_error): Handle if cuGetErrorString is not
	present.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263407 138bc75d-0d04-0410-961f-82ee72b054a4