Run XamlCompiler.exe as ARM64-native on Windows on ARM

[dennisameling]

Fixes #11237

Problem

XamlCompiler.exe is an AnyCPU / IL-only .NET Framework executable. On Windows on ARM, AnyCPU IL-only images launch under x64 emulation by default. Since the XAML compiler dominates WinUI build time, this emulation tax makes ARM64 builds much slower than they need to be. Issue #11237 measured the compiler at ~76% of build time, and forcing it ARM64-native via a per-machine registry hack gave a ~3.4× compiler speedup.

Fix

Declare the native architectures the compiler supports via an application manifest, and reference it from the project:

• New src/XamlCompiler/Exe/XamlCompiler.manifest with:

  <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2024/WindowsSettings">
    <supportedArchitectures>amd64 arm64</supportedArchitectures>
  </asmv3:windowsSettings>

• Microsoft.UI.Xaml.Markup.Compiler.Executable.csproj: <ApplicationManifest>XamlCompiler.manifest</ApplicationManifest>

On Windows 11, version 24H2 and later, the loader then launches the compiler ARM64-native instead of x64-emulated — no per-machine Image File Execution Options PreferredMachine registry workaround needed. This was the approach suggested by @driver1998 in the issue.

See the supportedArchitectures documentation.

Verification

Measured on the actual shipped XamlCompiler.exe (from the microsoft.windowsappsdk.winui package — PE32 / ILONLY / AnyCPU) on Windows 11 ARM64 (build 26200). Architecture read at image-load time via GetProcessInformation(ProcessMachineTypeInfo) from an ARM64-native parent process (representing the ARM64 build host):

Binary	Launch architecture
XamlCompiler.exe as shipped (no supportedArchitectures)	x64 EMULATED
Same binary with this manifest injected	ARM64 NATIVE

Notes:

• IsWow64Process2 is unreliable for detecting x64-on-ARM64 on current builds (it reports processMachine=UNKNOWN for a verified-x64 image); GetProcessInformation / Task Manager's Architecture column are the reliable signals.
• Process architecture is inherited, so the manifest delivers ARM64-native when the process launching the compiler is itself ARM64-native (the normal arm64 .NET SDK build) — which is exactly the scenario in XamlCompiler.exe is a .NET Framework AnyCPU binary and runs under x64 emulation on Windows on ARM, dominating WinUI build time #11237.
• amd64 arm64 (rather than arm64 alone) keeps x64 hosts running the compiler x64-native.

[driver1998]

MSBuild has a builtin flag named PreferNativeArm64 that does just this:

dotnet/msbuild#10060