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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> You beat me to it, I was planning to look into it this month 😉

In case @Berrysoft isn't familiar with adding tests, do you think you could look into adding a test for this change?

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