Add Mach-O CMAKE_EXECUTABLE_FORMAT detection

Code for this was prototyped when ELF detection was added long ago but
left commented out.  Use either MH_MAGIC or MH_CIGAM for the 32-bit
variant and use either or MH_MAGIC_64 or MH_CIGAM_64 for the 64-bit
variant.
20 jobs for machoFormat
latest
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