Build V8 from source by default in Bazel (#57832)

## Why

Source-built V8 archives and generated Rust bindings must agree on their ABI, including in debug builds. Linux platform selection must also distinguish GNU and musl when using LLVM platforms.

## What changed

- Always select source-built V8 archives and matching bindings for Bazel consumers, removing the `rusty_v8_from_source` flag.
- Use LLVM's libc-aware Linux selectors for archives, bindings, headers, and bindgen targets.
- Keep `V8_ENABLE_CHECKS` undefined in debug builds to match Rust's `HandleScope` layout, and set `V8_LOGGING_LEVEL=2` for debug verification.
- Include compiler-rt builtins in the ARM64 GNU/Linux archive to provide `__clear_cache`.
- Set the Intel macOS minimum version for `codex-code-mode-host` releases to 10.13 for V8's aligned allocation support.

## Testing

Always include the existing ignored CppGC ABI checks in Bazel `v8-poc` unit tests and remove their exclusion from Bazel CI.

GitOrigin-RevId: f698a47502154cdfb80febed02e4465d9f2d1e0c

.github/workflows/bazel.yml

@@ -96,7 +96,6 @@ jobs:
             # path. V8 consumers under `//codex-rs/...` still participate
             # transitively through `//...`.
             -//third_party/v8:all
-            -//codex-rs/v8-poc:v8-poc-unit-tests
           )
 
           bazel_wrapper_args=(