[libgomp, nvptx] Remove hard-coded const in nvptx_open_device

[libgomp, nvptx] Remove hard-coded const in nvptx_open_device

CU_DEVICE_ATTRIBUTE_MAX_REGISTERS_PER_MULTIPROCESSOR is defined in cuda driver
api version 6.0 and higher.

Currently nvptx_open_device uses a hard-coded constant instead.

This patch fixes that by:
- defining CU_DEVICE_ATTRIBUTE_MAX_REGISTERS_PER_MULTIPROCESSOR to the hardcoded
  constant at toplevel, if not present in cuda.h, and
- using CU_DEVICE_ATTRIBUTE_MAX_REGISTERS_PER_MULTIPROCESSOR in nvptx_open_device

Build on x86_64 with nvptx accelerator and reg-tested libgomp.

2018-08-08  Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c
	(CU_DEVICE_ATTRIBUTE_MAX_REGISTERS_PER_MULTIPROCESSOR): Define.
	(nvptx_open_device): Use
	CU_DEVICE_ATTRIBUTE_MAX_REGISTERS_PER_MULTIPROCESSOR.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263406 138bc75d-0d04-0410-961f-82ee72b054a4