[llvm] Revert "[Cygwin] Define LLVM_ABI for Cygwin (#143222)" (PR #144210)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

  I thought this should be applied after the CI script is fixed. If that's not necessary, I'll close this.

https://github.com/llvm/llvm-project/pull/144210