Windows: Enable win-arm64 for MSVC

This patch enabled win-arm64 as a new platform for Windows.
Platform query and documentation is updated accordingly.

winbuild/build.rst

@@ -24,7 +24,7 @@ Download and install:
 * `CMake 3.12 or newer <https://cmake.org/download/>`_
   (also available as Visual Studio component C++ CMake tools for Windows)
 
-* `NASM <https://www.nasm.us/pub/nasm/releasebuilds/?C=M;O=D>`_
+* x86/x64: `NASM <https://www.nasm.us/pub/nasm/releasebuilds/?C=M;O=D>`_
 
 Any version of Visual Studio 2017 or newer should be supported,
 including Visual Studio 2017 Community, or Build Tools for Visual Studio 2019.
@@ -42,8 +42,8 @@ behaviour of ``build_prepare.py``:
   If ``PYTHON`` is unset, the version of Python used to run
   ``build_prepare.py`` will be used. If only ``PYTHON`` is set,
   ``EXECUTABLE`` defaults to ``python.exe``.
-* ``ARCHITECTURE`` is used to select a ``x86`` or ``x64`` build. By default,
-  uses same architecture as the version of Python used to run ``build_prepare.py``.
+* ``ARCHITECTURE`` is used to select a ``x86``, ``x64`` or ``x86_arm64`` (for win-arm64) build.
+  By default, uses same architecture as the version of Python used to run ``build_prepare.py``.
   is used.
 * ``PILLOW_BUILD`` can be used to override the ``winbuild\build`` directory
   path, used to store generated build scripts and compiled libraries.

winbuild/build_prepare.py

@@ -1,4 +1,5 @@
 import os
+import platform
 import shutil
 import struct
 import subprocess
@@ -93,6 +94,7 @@ def cmd_msbuild(
 architectures = {
     "x86": {"vcvars_arch": "x86", "msbuild_arch": "Win32"},
     "x64": {"vcvars_arch": "x86_amd64", "msbuild_arch": "x64"},
+    "x86_arm64": {"vcvars_arch": "x86_arm64", "msbuild_arch": "ARM64"},
 }
 
 header = [
@@ -492,7 +494,10 @@ def build_pillow():
     python_dir = os.environ.get("PYTHON")
     python_exe = os.environ.get("EXECUTABLE", "python.exe")
     architecture = os.environ.get(
-        "ARCHITECTURE", "x86" if struct.calcsize("P") == 4 else "x64"
+        "ARCHITECTURE",
+        "x86_arm64"
+        if platform.machine() == "ARM64"
+        else ("x86" if struct.calcsize("P") == 4 else "x64"),
     )
     build_dir = os.environ.get("PILLOW_BUILD", os.path.join(winbuild_dir, "build"))
     sources_dir = ""