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

[via llvm-commits]

jeremyd2019 wrote:

I don't know - we'll see if somebody who does chimes in, but I wanted to clarify that this is really a CI bug.

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