[nvptx] Add nvptx_mach_vector_length, nvptx_mach_max_workers

[nvptx] Add nvptx_mach_vector_length, nvptx_mach_max_workers

The vector length and maximum number of workers are known compile-time.  Make
these easily available during code generation via new functions.

2019-01-03  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (MACH_VECTOR_LENGTH, MACH_MAX_WORKERS): Define.
	(init_axis_dim, nvptx_mach_max_workers, nvptx_mach_vector_length): New
	function.
	* config/nvptx/nvptx.h (struct machine_function): Add axis_dims.

From-SVN: r267558