[Windows] Update the Windows build layout to include the target architecture #129805

Background

See: https://flutter.dev/go/windows-arm64

Currently, Windows has the following build layout: build/windows/runner/Release/my_app.exe

This assumes an AMD64 target architecture.

Solution

Windows should be updated to have separate build directories for each target architectures. For example:

build/windows/x64/runner/Release/my_app.exe

Work

Note
This work item does not include targeting Arm64: Arm64 hosts will produce x64 executables.
Targeting Arm64 will be added by #129807

1. Update the Flutter Windows app template
   1. Replace hardcoded x64 with FLUTTER_TARGET_PLATFORM CMake variable. If FLUTTER_TARGET_PLATFORM is not set, its value should fallback to x64 for backwards compatibility.
   2. Update ephemeral directory path to use the target architecture's subfolder
2. Update the Flutter tool:
   1. Update CMake generation to set the target architecture explicitly using -A x64
   2. Update CMake build to set the FLUTTER_TARGET_PLATFORM CMake variable to x64
   3. Add a target architecture subfolder to the ephemeral directory
   4. Automatically migrate CMake files to match the new app template
   5. Automatically clean the build/windows/ directory if it contains builds that used the old layout
3. Update the Flutter docs:
   1. Update documentation that mentions the build output path
   2. Update the next release's breaking changes