[nvptx] Fix handling of extern var with flexible array member

[nvptx] Fix handling of extern var with flexible array member

2018-04-12  Tom de Vries  <tom@codesourcery.com>

	PR target/85296
	* config/nvptx/nvptx.c (flexible_array_member_type_p): New function.
	(nvptx_assemble_decl_begin): Add undefined param.  Declare undefined
	array with flexible array member as array without given dimension.
	(nvptx_assemble_undefined_decl): Set nvptx_assemble_decl_begin call
	argument for undefined param to true.

From-SVN: r259337