Cuda driver api functions cuLinkAddData and cuLinkCreate are available starting
version 5.5.  In version 6.5, they are remapped onto _v2 versions.

The dlopen interface of the libgomp nvptx plugin uses the _v2 versions, so it
won't work with a cuda driver with driver api version lower than 6.5.

This patch fixes the problem by testing for the presence of the _v2 versions,
and falling back to the original versions in case of absence of the _v2
versions.

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 (cuLinkAddData_v2, cuLinkCreate_v2): Declare using
	CUDA_ONE_CALL_MAYBE_NULL.
	* plugin/plugin-nvptx.c (cuLinkAddData, cuLinkCreate): Undef and declare.
	(cuLinkAddData_v2, cuLinkCreate_v2): Declare.
	(link_ptx): Fall back to cuLinkAddData/cuLinkCreate if the _v2 versions
	are not found.

From-SVN: r263408