[nvptx] Add PTX_WARP_SIZE

[nvptx] Add PTX_WARP_SIZE

Add PTX_WARP_SIZE constant and use it in nvptx_simt_vf.  The function
nvptx_simt_vf is used for OpenMP, and using PTX_WARP_SIZE here decouples the
OpenMP support from the PTX_VECTOR_LENGTH constant used in OpenACC support.

Build and reg-tested on x86_64 with nvptx accelerator.

2018-12-17  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (PTX_WARP_SIZE): Define.
	(nvptx_simt_vf): Return PTX_WARP_SIZE instead of PTX_VECTOR_LENGTH.

From-SVN: r267215