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

codex-rs/code-mode-host/BUILD.bazel

@@ -8,4 +8,6 @@ codex_rust_crate(
 
 release_binary(
     binary = ":codex-code-mode-host",
+    # V8 and its libc++ use aligned new and delete, available on macOS 10.13.
+    macos_x86_64_minimum_os = "10.13",
 )

codex-rs/v8-poc/BUILD.bazel

@@ -5,11 +5,8 @@ codex_rust_crate(
     crate_features = ["sandbox"],
     crate_name = "codex_v8_poc",
     deps_extra = ["@crates//:v8"],
-    # The ignored CppGC ABI checks require the native archive/binding pair.
-    unit_test_args = select({
-        "//third_party/v8:use_rusty_v8_prebuilts": [],
-        "//conditions:default": ["--include-ignored"],
-    }),
+    # Bazel always builds the native pair required by the CppGC ABI checks.
+    unit_test_args = ["--include-ignored"],
 )
 
 alias(