Add asserts on macOS/Windows so that they can't be anything but default

...for now?

cibuildwheel/windows.py

@@ -3,6 +3,7 @@ import shutil
 import subprocess
 import sys
 import tempfile
+import textwrap
 from os import PathLike
 from pathlib import Path
 from typing import Dict, List, NamedTuple, Optional, Sequence, Union
@@ -12,7 +13,7 @@ import toml
 
 from .environment import ParsedEnvironment
 from .logger import log
-from .util import (BuildOptions, BuildSelector, NonPlatformWheelError,
+from .util import (Architecture, BuildOptions, BuildSelector, NonPlatformWheelError,
                    download, get_build_verbosity_extra_flags, get_pip_script,
                    prepare_command)
 
@@ -201,6 +202,13 @@ def pep_518_cp35_workaround(package_dir: Path, env: Dict[str, str]) -> None:
 
 
 def build(options: BuildOptions) -> None:
+    if options.architectures != [Architecture.amd64, Architecture.x86]:
+        raise ValueError(textwrap.dedent(f'''
+            Invalid archs option {options.architectures}. Windows only supports 'amd64,x86' for the
+            moment. If you want to set emulation architectures on Linux, use CIBW_ARCHS_LINUX
+            instead.
+        '''))
+
     temp_dir = Path(tempfile.mkdtemp(prefix='cibuildwheel'))
     built_wheel_dir = temp_dir / 'built_wheel'
     repaired_wheel_dir = temp_dir / 'repaired_wheel'