Zig based cross-compiling toolchain

Major known issues:

• Zig seems to randomly fail with parallel builds with 'error: unable to build ... CRT file: BuildingLibCObjectFailed' This seems like race condition with the cache? ziglang/zig#13160 ziglang/zig#9711

• Rust and zig fight for the ownership of libcompiler_rt ziglang/zig#5320

Why zig for cross-compiling?

Zig can cross-compile out of the box to many target without having to bootstrap the whole cross-compiler and various libcs This means builds are very fast

TODO

• Generate meta.json with github actions
  • Rank zig versions and targets depending how well they do in tests
• Provide zigPkgs set with packages maintained in this repo
  • Packages that need major restructuring to compile
  • Minimal versions of existing packages (we are mostly interested in libs only)
  • Packages that do not exist yet in nixpkgs
  • Namespaced for different platforms (c, rust, etc...)
  • Generally zigPkgs is expected to compile and work while pkgs from nixpkg can be hit and miss

For other stuff, run things-to-do