Experimental cygwin host support

[workingjubilee]

While we do not yet formally support the cygwin target by distributing binaries, that is not because we do not wish for people to be able to build a rustc that uses cygwin as its host if they so please. I am opening this issue so that interested parties can track the progress on enabling a rustc to be built for cygwin. There's a few needed patches:

• compiler-builtins: Add cygwin support compiler-builtins#774
• libc: Add cygwin support libc#4279
• std: Add std support to cygwin target #137621

cc @Berrysoft

[Berrysoft]

The tracking issue for cargo: rust-lang/cargo#15442

[Berrysoft]

• Add cygwin support stacker#122
• feat: add cygwin support nagisa/rust_libloading#173
• Update nix Detegr/rust-ctrlc#131
• Is it possible to continue maintain version 0.2? rust-random/getrandom#653

[jeremyd2019]

In addition to cargo, there seems to be at least one other Cargo.toml that needs a version bump:

rust/src/tools/rust-analyzer/Cargo.toml

Line 119 in 462cc09

libloading = "0.8.6"