[libgomp, nvptx] Move device property sampling from nvptx_exec to nvptx_open

[libgomp, nvptx] Move device property sampling from nvptx_exec to nvptx_open

Move sampling of device properties from nvptx_exec to nvptx_open, and assume
the sampling always succeeds.  This simplifies the default dimension
initialization code in nvptx_open.

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

	* plugin/plugin-nvptx.c (struct ptx_device): Add warp_size,
	max_threads_per_block and max_threads_per_multiprocessor fields.
	(nvptx_open_device): Initialize new fields.
	(nvptx_exec): Use num_sms, and new fields.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
From-SVN: r262996