Add ARCH_INDEPENDENT flag in CMakeLists.txt

[astralaster]

If this package is flagged as ARCH_IDENPENDENT by cmake it is possible to consume the generated cmake files for compiling both 32bit and 64bit projects. Otherwise cmake will complain: CMake Error at CMakeLists.txt:12 (find_package): Could not find a configuration file for package "cereal" that is compatible with requested version "".

The following configuration files were considered but not accepted:

cmake/cereal/cerealConfig.cmake, version: 1.3.2 (64bit)

For more info about this flag look here: https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html