[llvm] [SystemZ][z/OS] Emit CELQMAIN for a module defining main (PR #226710)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand commented:

I'll leave it to @redstar to check the details of the emitted function.  One question: do we now need to change the default link command lines e.g. in clang to no longer link against this special object - so we don't get a duplicate definition?

https://github.com/llvm/llvm-project/pull/226710