[lld] [LLD][Cygwin] Add libcygwin to exclude from auto-export library list (PR #143118)

[Martin Storsjö via llvm-commits]

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

LGTM

When updating the intended commit message, it's recommended that you update the PR subject/description accordingly as well. When merging (with "squash and merge" as is the only alternative in llvm-project), the default commit message is based on the PR subject/description, not the individual commit messages. In this case I can take care to update it manually when merging though.

https://github.com/llvm/llvm-project/pull/143118