Visual Studio projects: Support arm64/aarch64 builds better

Update the NMake Makefiles used for generating the various sources be able to
cope with an ARM64 build environment, and update the project files, in
particular for Visual Studio 2017 (VisualStudioVersion 15.0) so that we can
pull in the system .lib's that were somehow excluded from the default list
on ARM64 builds.

Also, add a custom Directory.Build.props in win32/vs15 so that we do not
try to build with the Windows 8.1 SDK by defualt, which is not ready for ARM64
builds, but instead uses the appropriate Windows 10 SDK that supports this.

Update the README.win32 file to give people instructions on how ARM64 builds
can be carried out.

win32/detectenv-msvc.mak

@@ -21,6 +21,8 @@ _HASH=^#
     && ![echo PLAT=Win32 >> vercl.x] \
     && ![echo $(_HASH)elif defined(_M_AMD64) >> vercl.x] \
     && ![echo PLAT=x64 >> vercl.x] \
+    && ![echo $(_HASH)elif defined(_M_ARM64) >> vercl.x] \
+    && ![echo PLAT=arm64 >> vercl.x] \
     && ![echo $(_HASH)endif >> vercl.x] \
     && ![cl -nologo -TC -P vercl.x $(ERRNUL)]
 !include vercl.i