[libgomp, nvptx] Handle CUDA_ONE_CALL_MAYBE_NULL

[libgomp, nvptx] Handle CUDA_ONE_CALL_MAYBE_NULL

This patch adds handling of functions that may not be present in the cuda
driver.

Such a function can be declared using CUDA_ONE_CALL_MAYBE_NULL in cuda-lib.def,
it can be called with the usual convenience macros, but before calling its
presence needs to be tested using new macro CUDA_CALL_EXISTS.

When using the dlopen interface (PLUGIN_NVPTX_DYNAMIC == 1), we allow
non-present functions by allowing dlsym to return NULL.  Otherwise
(PLUGIN_NVPTX_DYNAMIC == 0) we declare the non-present function to be weak.

Build and reg-tested libgomp on x86_64 with nvidia accelerator, with and without
--disable-cuda-driver, in combination with a trigger patch that adds a
non-existing function foo to cuda-lib.def:
...
CUDA_ONE_CALL_MAYBE_NULL (foo)
...
and declares it in plugin-nvptx.c:
...
CUresult foo (void);
...
and then uses it in nvptx_init after the init_cuda_lib call:
...
  if (CUDA_CALL_EXISTS (foo))
    CUDA_CALL (foo);
...

Also build and reg-tested on x86_64 with nvidia accelerator, with and without
--disable-cuda-driver, in combination with a trigger patch that replaces all
CUDA_ONE_CALLs in cuda-lib.def with CUDA_ONE_CALL_MAYBE_NULL, and guards two
CUDA_CALLs with CUDA_CALL_EXISTS, one for a regular fn, and one for a fn that is
a define in cuda/cuda.h.

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

	* plugin/plugin-nvptx.c (DO_PRAGMA): Define.
	(struct cuda_lib_s): Add def/undef of CUDA_ONE_CALL_MAYBE_NULL.
	(init_cuda_lib): Add new param to CUDA_ONE_CALL_1.  Add arg to
	corresponding call in CUDA_ONE_CALL.  Add def/undef of
	CUDA_ONE_CALL_MAYBE_NULL.
	(CUDA_CALL_EXISTS): Define.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263346 138bc75d-0d04-0410-961f-82ee72b054a4