COMP: Constrain x86_64 optimization flags according to CMAKE_SYSTEM_PROCESSOR
COMP: Constrain x86_64 optimization flags according to CMAKE_SYSTEM_PROCESSOR

CMAKE_SYSTEM_PROCESSOR defines the target compilation architecture. When
we want x86_64 optimizations, e.g. -march=corei7, only add these flags
when building for this architecture. On Linux/macOS (GCC/AppleClang),
CMAKE_SYSTEM_PROCESSOR is "x86_64" and on Windows (Visual Studio) it is
"AMD64".

richardbeare/ITK,vfonov/ITK,Kitware/ITK,LucasGandel/ITK,vfonov/ITK,BRAINSia/ITK,Kitware/ITK,Kitware/ITK,vfonov/ITK,vfonov/ITK,hjmjohnson/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,richardbeare/ITK,BRAINSia/ITK,hjmjohnson/ITK,LucasGandel/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK,richardbeare/ITK,vfonov/ITK,Kitware/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,BRAINSia/ITK,hjmjohnson/ITK,LucasGandel/ITK,LucasGandel/ITK,BRAINSia/ITK,Kitware/ITK,thewtex/ITK,LucasGandel/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,hjmjohnson/ITK,BRAINSia/ITK,vfonov/ITK,richardbeare/ITK,richardbeare/ITK,LucasGandel/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,thewtex/ITK,vfonov/ITK