[libgomp, nvptx] Minimize lifetime of CUDA_ONE_CALL defines

[libgomp, nvptx] Minimize lifetime of CUDA_ONE_CALL defines

This patch makes sure that the lifetimes of the CUDA_ONE_CALL macro (which is
defined twice in plugin-nvptx.c) are minimized, to make it obvious that the
definitions are used only in the lib-cuda.def include.

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

2018-08-07  Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (struct cuda_lib_s, init_cuda_lib): Put
	CUDA_ONE_CALL defines right before the cuda-lib.def include, and the
	corresponding undefs right after.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263345 138bc75d-0d04-0410-961f-82ee72b054a4