feat: use libc without cc-toolchain

[aherrmann]

The Zig compiler bundles its own libc(++). rules_zig exposes these using the targets @rules_zig//zig/lib:libc(++). However, so far these were implemented as cc_library targets, meaning that depending on them incurred a dependency on the cc-toolchain, even if the target was only used to forward a -lc(++) linker flag.

This change removes that dependency. These targets are now generated using a custom rule that is still compatible with the cc rules, but no longer requires a cc-toolchain. This greatly simplifies cross-compilation use-cases with libc(++) dependencies. For example

$ cd e2e/workspace
$ bazel run //include-dependencies/zig-std-include:binary --platforms=//configure-target:x86_64-windows --run_under=wine64

---

• Rule to set C linkopts w/o cc toolchain
• Remove dummy cc toolchain from integration test