[nvptx, offloading] Determine default workers at runtime

[nvptx, offloading] Determine default workers at runtime

Currently, if the user doesn't specify the number of workers for an openacc
region, the compiler hardcodes it to a default value.

This patch removes this functionality, such that the libgomp runtime can decide
on a default value.

2018-07-30  Cesar Philippidis  <cesar@codesourcery.com>
	    Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (PTX_GANG_DEFAULT): Rename to ...
	(PTX_DEFAULT_RUNTIME_DIM): ... this.
	(nvptx_goacc_validate_dims): Set default worker and gang dims to
	PTX_DEFAULT_RUNTIME_DIM.
	(nvptx_dim_limit): Ignore GOMP_DIM_WORKER.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
From-SVN: r263060