llvm, clang, libgit2, curl, rust builds

[Jeremy Drake]

I made an effort to make github workflows to bootstrap rust, and in the
process llvm/clang, for Cygwin.

https://github.com/jeremyd2019/cygwin-rust-bootstrap/blob/main/.github/workflows/bootstrap_llvm.yml
https://github.com/jeremyd2019/cygwin-rust-bootstrap/actions/runs/15260152503

The LLVM packages are input to
https://github.com/jeremyd2019/cygwin-rust-bootstrap/blob/main/.github/workflows/bootstrap_rust.yml
https://github.com/jeremyd2019/cygwin-rust-bootstrap/actions/runs/15292314203

which builds a patched curl and updated libgit2, cross-compiles rust
x86_64-pc-windows-gnu -> x86_64-pc-cygwin, and then uses the
cross-compiled rust to build native x86_64-pc-cygwin