When building for multiple architectures on OSX, it's necessary to use compiler macros to conditionally include architecture-specific code rather than conditionally including architecture-specific assembly files via cmake. See https://developer.apple.com/documentation/apple-silicon/building-a-universal-macos-binary and https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_ARCHITECTURES.html for more details. Co-authored-by: Zoraaver Singh <zoraaver@amazon.co.uk>
