nvptx, libgcc: Stub unwinding implementation

nvptx, libgcc: Stub unwinding implementation

Adding stub '_Unwind_Backtrace', '_Unwind_GetIPInfo' functions is necessary
for linking libbacktrace, as a normal (non-'LIBGFOR_MINIMAL') configuration
of libgfortran wants to do, for example.

The file 'libgcc/config/nvptx/unwind-nvptx.c' is copied from
'libgcc/config/gcn/unwind-gcn.c'.

libgcc/ChangeLog:

	* config/nvptx/t-nvptx: Add unwind-nvptx.c.
	* config/nvptx/unwind-nvptx.c: New file.

Co-authored-by: Andrew Stubbs <ams@gcc.gnu.org>