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

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM, thanks.

Ideally, any functional change like this would warrant a change to a test case though. One way of identifying an existing test case relating to this, would be to e.g. break the existing code and run `ninja check-llvm` or similar, and see which tests now fail. That would show e.g. this list of tests:
```
  LLVM :: CodeGen/AArch64/mingw-refptr.ll
  LLVM :: CodeGen/ARM/Windows/mingw-refptr.ll
  LLVM :: CodeGen/ARM/Windows/pic.ll
  LLVM :: CodeGen/ARM/emutls_generic.ll
  LLVM :: CodeGen/ARM/win32-ssp.ll
  LLVM :: CodeGen/X86/PR40322.ll
  LLVM :: CodeGen/X86/br-fold.ll
  LLVM :: CodeGen/X86/mingw-refptr.ll
  LLVM :: CodeGen/X86/stack-protector.ll
  LLVM :: CodeGen/X86/win32-ssp.ll
  LLVM :: CodeGen/X86/win64-byval.ll
```
So it might be possible to copy some of these, or add `RUN` lines within the existing ones, for doing the same test for a cygwin target.

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