Attached is the updated patch. Main changes:
- File names shown that violate the requires-clause-must-be-same requirement
   Taken from the offload_vars/funcs context (if available), otherwise
   (that's no 'omp target'/'omp declare target' but just 'omp target update/data'
   in the TU), the *.o file name is used.
(thanks to richi + jakub for the susggestions!)
- Uses GOMP_register_var to pass the mask to libgomp
(and no longer a weak variable)
- 'omp declare target' is not regarded as being used -> pending OpenMP lang spec clarification
- 'omp target update' is for C/C++
- Properly handle is used by-target constucts for Fortran
- Save requires (and empty offload table) in the *.o file, even if it is only
   using 'omp target (enter/exit) data'

Thanks goes to Jakub for many useful suggestions!

Tested without offloading configured and with nvptx and amdgcn offloading (all on x86_64-gnu-linux).

OK? Or does anyone have more useful suggestions?

Tobias
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