Introduce MSW ARM64 support

This is a preliminary ARM64 platform support for wxWidgets at "it
compiles" stage. This will allow building and testing wxWidgets based
apps for oncoming Windows 10 ARM64.

Requirements:
- Visual Studio 2017 Update 4 or later with Visual C++ compilers and
  libraries for ARM64 component installed

Building:
1. Open command prompt.
2. Change directory to build\msw subfolder.
3. Run "C:\Program Files (x86)\Microsoft Visual
   Studio\2017\Community\VC\Auxiliary\Build\vcvarsamd64_arm64.bat" once.
4. Use `nmake TARGET_CPU=ARM64 ...` to build required flavor of wxWidget
   libraries.

Notes:
1. Building of *.sln/*.vcxproj files does not support ARM64 yet. This
   requires to hardcode Windows SDK to 10.0.15063.0 or later in
   *.vcxproj files, which would render them non-compilable in older
   Visual Studio versions. Microsoft is aware of this issue and is
   planning a fix in the next version of Visual Studio.
2. wxmsw31ud_gl.dll does not build yet. Awaiting Microsoft to deliver
   missing opengl32.lib for ARM64. Please, specify USE_OPENGL=0.

Closes https://github.com/wxWidgets/wxWidgets/pull/923

docs/changes.txt

@@ -125,8 +125,9 @@ wxGTK:
 
 wxMSW:
 
+- Add experimental support for Windows 10/ARM64 platform (Simon Rozman).
 - Fix hang after clearing wxTAB_TRAVERSAL style on a window with children.
-- Fix handling of AUX2 mouse button events (Trylz).
+- Fix handling of AUX2 mouse button events (Timon Rozmanrylz).
 - Fix saving/restoring window position for maximized windows.
 - Fix stack corruption when using wxStackWalker (srfisk).
 - Fix positioning windows at positions >= SHORT_MAX (Cătălin Răceanu).

docs/msw/install.txt

@@ -455,7 +455,7 @@ DEBUG_RUNTIME_LIBS=1
   usable .pdb files with debug information) and this setting makes it
   possible.
 
-TARGET_CPU=X64|IA64
+TARGET_CPU=X64|ARM64|IA64
   (VC++ only.) Set this variable to build for x86_64 systems. If unset, x86
   build is performed.