nvptx/mkoffload.cc: Add dummy proc for OpenMP rev-offload table [PR108098]

Seemingly, the ptx JIT of CUDA <= 10.2 replaces function pointers in global
variables by NULL if a translation does not contain any executable code. It
works with CUDA 11.1.  The code of this commit is about reverse offload;
having NULL values disables the side of reverse offload during image load.

Solution is the same as found by Thomas for a related issue: Adding a dummy
procedure. Cf. the PR of this issue and Thomas' patch
"nvptx: Support global constructors/destructors via 'collect2'"
https://gcc.gnu.org/pipermail/gcc-patches/2022-December/607749.html



As that approach also works here:

Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>

gcc/
	PR libgomp/108098

	* config/nvptx/mkoffload.cc (process): Emit dummy procedure
	alongside reverse-offload function table to prevent NULL values
	of the function addresses.

(cherry picked from commit 4359724c)