Add Mach-O CMAKE_EXECUTABLE_FORMAT detection

Currently it is not possible to use CMAKE_EXECUTABLE_FORMAT to identify the Mach-O format. Its unclear why this code was disabled before. This MR is changing the approach to use either MH_MAGIC or MH_CIGAM for the 32-bit variant or MH_MAGIC_64 or MH_CIGAM_64 for the 64-bit variant