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

include/wx/defs.h

@@ -3033,6 +3033,8 @@ typedef const void* WXWidget;
     #pragma comment(linker, WX_CC_MANIFEST("x86"))
 #elif defined _M_X64
     #pragma comment(linker, WX_CC_MANIFEST("amd64"))
+#elif defined _M_ARM64
+    #pragma comment(linker, WX_CC_MANIFEST("arm64"))
 #elif defined _M_IA64
     #pragma comment(linker, WX_CC_MANIFEST("ia64"))
 #else