[codex] Provide ARM64 MinGW powl compatibility support (#27323)

## Why

Windows ARM64 uses 64-bit `long double`, but the LLVM MinGW Bazel
configuration omits the upstream `powl` compatibility source and does
not link the `mingwex` archive that owns it. Cross-linking the release
binary therefore fails with an unresolved `powl` symbol.

## What changed

Patch the LLVM module to compile `math/arm-common/powl.c` into the ARM64
MinGW extension sources and add `-lmingwex` to the Windows toolchain
defaults.

## Validation

- `just bazel-lock-check`

Stack: 3 of 6. Depends on #27322.

MODULE.bazel

@@ -11,6 +11,7 @@ single_version_override(
     patch_strip = 1,
     patches = [
         "//patches:llvm_rusty_v8_custom_libcxx.patch",
+        "//patches:llvm_windows_arm64_powl.patch",
         "//patches:llvm_windows_symlink_extract.patch",
     ],
 )