Build: Use correct SIMD exts w/VStudio IDE + Arm64

When configuring a Visual Studio IDE build and passing -A arm64 to
CMake, CMAKE_SYSTEM_PROCESSOR will be amd64, so we should set CPU_TYPE
based on the value of CMAKE_GENERATOR_PLATFORM rather than the value of
CMAKE_SYSTEM_PROCESSOR.

CMakeLists.txt

@@ -67,6 +67,10 @@ elseif(CMAKE_SYSTEM_PROCESSOR_LC MATCHES "ppc*" OR
 else()
   set(CPU_TYPE ${CMAKE_SYSTEM_PROCESSOR_LC})
 endif()
+if(MSVC_IDE AND CMAKE_GENERATOR_PLATFORM MATCHES "arm64")
+  set(CPU_TYPE arm64)
+endif()
+
 message(STATUS "${BITS}-bit build (${CPU_TYPE})")