[nvptx] Use CUDA driver API to select default runtime launch geometry

[nvptx] Use CUDA driver API to select default runtime launch geometry

The CUDA driver API starting version 6.5 offers a set of runtime functions to
calculate several occupancy-related measures, as a replacement for the occupancy
calculator spreadsheet.

This patch adds a heuristic for default runtime launch geometry, based on the
new runtime function cuOccupancyMaxPotentialBlockSize.

Build on x86_64 with nvptx accelerator and ran libgomp testsuite.

2018-08-13  Cesar Philippidis  <cesar@codesourcery.com>
	    Tom de Vries  <tdevries@suse.de>

	PR target/85590
	* plugin/cuda/cuda.h (CUoccupancyB2DSize): New typedef.
	(cuOccupancyMaxPotentialBlockSize): Declare.
	* plugin/cuda-lib.def (cuOccupancyMaxPotentialBlockSize): New
	CUDA_ONE_CALL_MAYBE_NULL.
	* plugin/plugin-nvptx.c (CUDA_VERSION < 6050): Define
	CUoccupancyB2DSize and declare
	cuOccupancyMaxPotentialBlockSize.
	(nvptx_exec): Use cuOccupancyMaxPotentialBlockSize to set the
	default num_gangs and num_workers when the driver supports it.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
From-SVN: r263505