Avoid unused Clang runtime-selection flags in rustc links

[ghorn]

Rust links inherit --unwindlib=none and -rtlib=compiler-rt from LLVM Cc toolchains. With rustc's default linker libraries disabled (-nodefaultlibs), Clang does not consume these automatic runtime-selection options and rustc reports linker_messages warnings for each binary, including host tools.

Omit these exact options (including their single/double-dash aliases) from known Clang Linux/macOS rustc links when default libraries are disabled. Honor flag precedence and preserve the options when defaults are enabled, build-script/response-file/opaque flags are present, or a custom target or driver makes the configuration uncertain. Explicit runtime libraries and search paths remain intact. Filtering stays inside the existing path-mapped argument expansion; ordinary C/C++ links are unaffected.

Validation on macOS arm64 with Bazel 9.2.0:

• 13 tests passed across //test/unit/runtime_linker_flags:all, //test/unit/rustc_flag_ordering:all, and //test/unit/extra_rustc_flags:all. The new tests inspect real LLVM link actions for both Linux and macOS, covering defaults disabled, enabled, and disabled by the final override.
• Built and ran the macOS fixture with --extra_toolchains=@llvm//toolchain:all; neither unused-option warning was emitted, including during the process-wrapper host build.
• Linux LLVM cross-link validation is limited by missing libgcc_s in this checkout. The same link fails on unchanged main; the baseline emits both unused-option diagnostics, while the patched link removes them. Linux action-level regressions pass.
• Buildifier checks and git diff --check passed.

Assisted-by: OpenAI Codex