[libgomp, nvptx] Calculate default dims per device

[libgomp, nvptx] Calculate default dims per device

The default dimensions are calculated using per-device properties, but
initialized once and used on all devices.

This patch fixes this problem by introducing per-device default dimensions.

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

	* plugin/plugin-nvptx.c (struct ptx_device): Add default_dims field.
	(nvptx_open_device): Init default_dims for device.
	(nvptx_exec): Use default_dims from device.

From-SVN: r263061