Fix formatting of output C++ toolchain for macOS.

Previously the generated `external/local_config_cc/BUILD` was a bit off,
which triggerred buildifier when trying to reuse it for a custom
CROSSTOOL.

Before:

```starlark
[
    cc_toolchain_config(
        name = (arch if arch != "armeabi-v7a" else "stub_armeabi-v7a"),
        compiler = "compiler",
        cpu = arch,
        cxx_builtin_include_directories = [        "/Applications/Xcode-11.7.0.app/Contents/Developer",
        "/Applications/Xcode-12.0.0.app/Contents/Developer",
        "/Applications/",],
        tool_paths_overrides = {},
    )
    for arch in OSX_TOOLS_ARCHS
]
```

After:

```starlark
[
    cc_toolchain_config(
        name = (arch if arch != "armeabi-v7a" else "stub_armeabi-v7a"),
        compiler = "compiler",
        cpu = arch,
        cxx_builtin_include_directories = [
            "/Applications/Xcode-11.7.0.app/Contents/Developer",
            "/Applications/Xcode-12.0.0.app/Contents/Developer",
            "/Applications/",
        ],
        tool_paths_overrides = {},
    )
    for arch in OSX_TOOLS_ARCHS
]
```

Tested by running `bazel sync --configure` and checking the contents of
`external/local_config_cc/BUILD`.

Closes #12157.

PiperOrigin-RevId: 335613727