CMakeLists: add default "x64-windows" arch for Visual Studio

In Git-for-Windows, work on using ARM64 has progressed. The
commit 2d94b77 (cmake: allow building for Windows/ARM64, 2020-12-04)
failed to notice that /compat/vcbuild/vcpkg_install.bat will default to
using the "x64-windows" architecture for the vcpkg installation if not set,
but CMake is not told of this default. Commit 635b6d9 (vcbuild: install
ARM64 dependencies when building ARM64 binaries, 2020-01-31) later updated
vcpkg_install.bat to accept an arch (%1) parameter, but retained the default.

This default is neccessary for the use case where the project directory is
opened directly in Visual Studio, which will find and build a CMakeLists.txt
file without any parameters, thus expecting use of the default setting.

Also Visual studio will generate internal .sln solution and .vcxproj project
files needed for some extension tools. Inform users of the additional
.sln/.vcxproj generation.

** How to test:
 rm -rf '.vs' # remove old visual studio settings
 rm -rf 'compat/vcbuild/vcpkg' # remove any vcpkg downloads
 rm -rf 'contrib/buildsystems/out' # remove builds & CMake artifacts
 with a fresh Visual Studio Community Edition, File>>Open>>(git *folder*)
   to load the project (which will take some time!).
 check for successful compilation.
The implicit .sln (etc.) are in the hidden .vs directory created by
Visual Studio.

Signed-off-by: Philip Oakley <philipoakley@iee.email>

contrib/buildsystems/CMakeLists.txt

@@ -71,6 +71,10 @@ if(USE_VCPKG)
 		message("Initializing vcpkg and building the Git's dependencies (this will take a while...)")
 		execute_process(COMMAND ${CMAKE_SOURCE_DIR}/compat/vcbuild/vcpkg_install.bat ${VCPKG_ARCH})
 	endif()
+	if(NOT EXISTS ${VCPKG_ARCH})
+		message("VCPKG_ARCH: unset, using 'x64-windows'")
+		set(VCPKG_ARCH "x64-windows") # default from vcpkg_install.bat
+	endif()
 	list(APPEND CMAKE_PREFIX_PATH "${VCPKG_DIR}/installed/${VCPKG_ARCH}")
 
 	# In the vcpkg edition, we need this to be able to link to libcurl