[Offload] Fix the offloading wrapper when merged multiple times. (#79231)

Summary:
The offloading wrapper is a object file that contains code necessary to
register offloading entries for the given runtime. Currently, we
expected only one of these to be present when we make the final
executable. However, in the case of redistributable linking with `-r` we
can end up with multiple of these being generated before finally
creating the executable.

This patch simply changes the defintiions of these globals to be
mergable. This allows multiples of these to participate in a single link
job. For ELF, we just make the dummy variable internal and used so it
sets up the section as expected. For COFF we make the entries weak_odr
so they merge to a single symbol