Add platform-specific CIBW_ARCHS options.

Remove 'auto' as a possible Architecture enum value. This is resolved
at option parse time.

cibuildwheel/linux.py

@@ -1,4 +1,3 @@
-import platform
 import re
 import subprocess
 import sys
@@ -75,14 +74,6 @@ def get_python_configurations(
     # skip builds as required
     target_archs = architectures
 
-    if Architecture.auto in architectures:
-        target_archs.remove(Architecture.auto)
-        native_architecture = Architecture(platform.machine())
-        target_archs.append(native_architecture)
-        # x86_64 machines can run i686 docker containers
-        if native_architecture == Architecture.x86_64:
-            target_archs.append(Architecture.i686)
-
     return [
         c for c in python_configurations
         if matches_platform(c.identifier, target_archs)