Rollup merge of #134999 - Berrysoft:dev/new-cygwin-target, r=chenyukang,workingjubilee

Add cygwin target.

This PR simply adds cygwin target together with msys2 target, based on ````@ookiineko```` 's (the account has been deleted) [work](https://github.com/ookiineko-cygport/rust) on cygwin target. My full work is here: rust-lang/rust@master...Berrysoft:rust:dev/cygwin

I have succeeded in building a new rustc for cygwin target, and eventually distributed a new version of [fish-shell](https://github.com/Berrysoft/fish-shell/releases) (rewritten by Rust) for MSYS2.

I will open a new PR to fix std if this PR is accepted.

Author: jhpratt