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: https://github.com/rust-lang/rust/compare/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.

compiler/rustc_target/src/spec/mod.rs

@@ -2013,6 +2013,7 @@ supported_targets! {
     ("riscv64imac-unknown-nuttx-elf", riscv64imac_unknown_nuttx_elf),
     ("riscv64gc-unknown-nuttx-elf", riscv64gc_unknown_nuttx_elf),
 
+    ("x86_64-pc-cygwin", x86_64_pc_cygwin),
 }
 
 /// Cow-Vec-Str: Cow<'static, [Cow<'static, str>]>
@@ -2994,8 +2995,8 @@ impl Target {
         );
         check_eq!(
             self.is_like_windows,
-            self.os == "windows" || self.os == "uefi",
-            "`is_like_windows` must be set if and only if `os` is `windows` or `uefi`"
+            self.os == "windows" || self.os == "uefi" || self.os == "cygwin",
+            "`is_like_windows` must be set if and only if `os` is `windows`, `uefi` or `cygwin`"
         );
         check_eq!(
             self.is_like_wasm,