[llvm] [LLVM] adjust lit.cfg.py for Cygwin (PR #151416)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo commented:

Looking mostly good, but I do find the `-elf` case curious, why it should fail more on cygwin than on native windows.

https://github.com/llvm/llvm-project/pull/151416