rust: ring build script finds no clang on windows_arm64 under --//cc/toolchains:compiler=msvc #9224

Follow-up from #6588 (part of #4825). With --//cc/toolchains:compiler=msvc on windows_arm64, //cli/cli:dx fails in the ring 0.17.14 build script: cc-rs failed to find tool "clang" (evidence run https://github.com/devhusdk/rules_dx/actions/runs/42127196421). ring's build.rs needs clang (or clang-cl with our patch) for aarch64 Windows assembly (briansmith/ring#2673), and the MSVC toolchain only has cl.exe. The default llvm toolchain builds it fine (CI preflight windows_arm64, run 42363337317 job 128376092757).

Where

• MODULE.bazel:1587-1599 (ring crate.annotation with clang_cl_aarch64_windows.patch) and MODULE.bazel:1626-1639 (ring crate.annotation_select for aarch64-pc-windows-msvc).
• rust/tools/msvc_paths/BUILD.bazel (pattern for an exec tool passed through build_script_tools).

Fix

• In the ring annotation_select for aarch64-pc-windows-msvc, add CC_aarch64_pc_windows_msvc = $(execpath <hermetic LLVM clang-cl>) and add that label to build_script_tools, the same way msvc_lib is passed as AR_*. This is harmless under the default llvm compiler, which already uses clang-cl.
• Prove it: dispatch evidence.yml with platforms=["windows_arm64"], flags=--//cc/toolchains:compiler=msvc, scope //cli/cli:dx. Post the run URL here and on ci: qualify compiler backends on clean hosts and publish compiler-specific support evidence #4825.
• If clang-cl cannot be exposed as an exec tool, record ring on windows_arm64 needs llvm as a known gap in the ci: qualify compiler backends on clean hosts and publish compiler-specific support evidence #4825 compiler table, and close this issue.