[libgomp, nvptx] Add error with recompilation hint for launch failure

[libgomp, nvptx] Add error with recompilation hint for launch failure

Currently, when a kernel is lauched with too many workers, it results in a cuda
launch failure.  This is triggered f.i. for parallel-loop-1.c at -O0 on a Quadro
M1200.

This patch detects this situation, and errors out with a hint on how to fix it.

Build and reg-tested on x86_64 with nvptx accelerator.

2018-07-26  Cesar Philippidis  <cesar@codesourcery.com>
	    Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (nvptx_exec): Error if the hardware doesn't have
	sufficient resources to launch a kernel, and give a hint on how to fix
	it.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
From-SVN: r262997