build: allow control over the Windows arches to build

This allows the user to specify the architectures that they would want
to build for Windows.  If it is not specified, then all known variants
(ARM and x86 32-bit and 64-bit) will be built.

CMakeLists.txt

@@ -775,7 +775,10 @@ endif()
 # Should we cross-compile the standard library for Windows?
 is_sdk_requested(WINDOWS swift_build_windows)
 if(swift_build_windows AND NOT "${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
-  configure_sdk_windows(WINDOWS "Windows" "msvc" "aarch64;armv7;i686;x86_64")
+  if("${SWIFT_SDK_WINDOWS_ARCHITECTURES}" STREQUAL "")
+    set(SWIFT_SDK_WINDOWS_ARCHITECTURES aarch64;armv7;i686;x86_64)
+  endif()
+  configure_sdk_windows(WINDOWS "Windows" "msvc" "${SWIFT_SDK_WINDOWS_ARCHITECTURES}")
 endif()
 
 if("${SWIFT_SDKS}" STREQUAL "")