CMAKE_SYSTEM_PROCESSOR is broken on Windows

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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According to it's description CMAKE_SYSTEM_PROCESSOR should have value of "The name of the CPU CMake is building for". On Windows x64 hosts it always has value of AMD64 no matter which architecture you are building for: x86/x64 or arm. I have a strong opinion that this behavior should be fixed cause otherwise this var is completely useless on windows. And yes I know there is a CMAKE_CL_64 var for determining what arch we are building for. But it looks like a x64-only dirty hack not a generic solution