[libgomp, nvptx] Note that cuGetErrorString is in CUDA_VERSION >= 6000

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

This patch:
- removes a comment saying the declaration is not available in cuda.h 6.0
- fixes the presence test to use CUDA_VERSION < 6000
- moves the declaration to toplevel

Build on x86_64 with nvptx accelerator and reg-tested libgomp.

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

	* plugin/plugin-nvptx.c (cuda_error): Move declaration of cuGetErrorString ...
	(cuGetErrorString): ... here.  Guard with CUDA_VERSION < 6000.

From-SVN: r263405

libgomp/ChangeLog

+2018-08-08  Tom de Vries  <tdevries@suse.de>
+
+	* plugin/plugin-nvptx.c (cuda_error): Move declaration of cuGetErrorString ...
+	(cuGetErrorString): ... here.  Guard with CUDA_VERSION < 6000.
+
 2018-08-07  Tom de Vries  <tdevries@suse.de>
 
 	* plugin/plugin-nvptx.c (DO_PRAGMA): Define.

libgomp/plugin/plugin-nvptx.c

@@ -49,6 +49,10 @@
 #include <assert.h>
 #include <errno.h>
 
+#if CUDA_VERSION < 6000
+extern CUresult cuGetErrorString (CUresult, const char **);
+#endif
+
 #define DO_PRAGMA(x) _Pragma (#x)
 
 #if PLUGIN_NVPTX_DYNAMIC
@@ -156,11 +160,6 @@ init_cuda_lib (void)
 static const char *
 cuda_error (CUresult r)
 {
-#if CUDA_VERSION < 7000
-  /* Specified in documentation and present in library from at least
-     5.5.  Not declared in header file prior to 7.0.  */
-  extern CUresult cuGetErrorString (CUresult, const char **);
-#endif
   const char *desc;
 
   r = CUDA_CALL_NOCHECK (cuGetErrorString, r, &desc);