[ATfE] Ensure we create universal binaries on MacOS builds (#240)

This updates the ATfE's CMake files to ensure universal binaries are
created for MacOS builds of the toolchain. This is done by setting the
`CMAKE_OSX_ARCHITECTURE` variable to cover both `x86_64` and `arm64`
architecture options.

(cherry picked from commit b65bfcf)

Author: pratlucas

arm-software/embedded/CMakeLists.txt

@@ -250,6 +250,8 @@ if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Release CACHE BOOL "" FORCE)
 endif()
 
+set(CMAKE_OSX_ARCHITECTURES arm64;x86_64 CACHE STRING "")
+
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
 if(NOT CMAKE_C_COMPILER_LAUNCHER AND NOT CMAKE_CXX_COMPILER_LAUNCHER)