nvptx: Prevent emitting duplicate declarations for '__nvptx_stacks', '__nvptx_uni'

nvptx: Prevent emitting duplicate declarations for '__nvptx_stacks', '__nvptx_uni'

As I have reported to Nvidia in 2022-12-01 'NVIDIA Incident Report (3891704):
ptxas: Duplicate declaration error: "cannot be resolved by a '.static'"',
'ptxas' has an inscrutable error mode for duplicate declarations:

    ptxas softstack-decl-1.o, line 11; error   : '.extern' variable '__nvptx_stacks' cannot be resolved by a '.static'
    ptxas fatal   : Ptx assembly aborted due to errors
    nvptx-as: ptxas returned 255 exit status

    ptxas uniform-simt-decl-1.o, line 12; error   : '.extern' variable '__nvptx_uni' cannot be resolved by a '.static'
    ptxas fatal   : Ptx assembly aborted due to errors
    nvptx-as: ptxas returned 255 exit status

This is inscrutable, because (a) what is "cannot be resolved by a '.static'"
supposed to tell me (there is no '.static' in PTX?), and (b) why arent't
repeated declaration just verified to match the first, but otherwise a no-op
(like in other programming languages)?

	gcc/
	* config/nvptx/nvptx.cc (nvptx_assemble_undefined_decl): Notice
	'__nvptx_stacks', '__nvptx_uni' declarations.
	(nvptx_file_end): Don't emit duplicate declarations for those.
	gcc/testsuite/
	* gcc.target/nvptx/softstack-decl-1.c: Make 'dg-do assemble',
	adjust.
	* gcc.target/nvptx/uniform-simt-decl-1.c: Likewise.