[llvm] [Cygwin] Fix global variable dll import (PR #121439)

[Mateusz Mikuła via llvm-commits]

mati865 wrote:

> Thank you. I'm really not familiar with adding tests. If @mati865 has a larger plan including this patch, I think it's also OK. I just wanted to solve the problem.

Please go ahead. My work is still in an early stage.

https://github.com/llvm/llvm-project/pull/121439