nvptx: Fix up nvptx build against latest libstdc++ [PR100375]

nvptx: Fix up nvptx build against latest libstdc++ [PR100375]

The r12-220-gd96db15967e78d7cecea3b1cf3169ceb924678ac change
deprecated some non-standard std::pair constructors and that apparently
broke nvptx.c build, where pseudo_node_t is std::pair<struct basic_block_def *, int>
and so nullptr (or NULL) needs to be used for the first argument of the
ctors instead of 0.

2021-05-02  Jakub Jelinek  <jakub@redhat.com>

	PR target/100375
	* config/nvptx/nvptx.c (nvptx_sese_pseudo): Use nullptr instead of 0
	as first argument of pseudo_node_t constructors.