[libgomp, nvptx] Handle per-function max-threads-per-block in default dims

[libgomp, nvptx] Handle per-function max-threads-per-block in default dims

Currently parallel-loop-1.c fails at -O0 on a Quadro M1200, because one of the
kernel launch configurations exceeds the resources available in the device, due
to the default dimensions chosen by the runtime.

This patch fixes that by taking the per-function max_threads_per_block into
account when using the default dimensions.

2018-07-30  Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (MIN, MAX): Redefine.
	(nvptx_exec): Ensure worker and vector default dims don't exceed
	targ_fn->max_threads_per_block.

From-SVN: r263062