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/msw/genrcdefs.h

@@ -27,6 +27,10 @@ EMIT(#define WX_CPU_AMD64)
 EMIT(#define WX_CPU_ARM)
 #endif
 
+#ifdef _M_ARM64
+EMIT(#define WX_CPU_ARM64)
+#endif
+
 #if defined _M_IA64 || defined __ia64__
 EMIT(#define WX_CPU_IA64)
 #endif