[llvm] Use XMACROS for MachO platforms. (#69262)

This change adds the PLATFORM XMACRO to simplify the addition of new MachO
platforms and reduce the number of required changes. Many of the changes needed
for adding a new platform are mechanical, such as adding new cases to a switch
statement. This will help streamline the process by consolidating much of the
necessary information into the MachO.def file.