Add support for Mach-O executables

[FinnWilkinson]

SimEng currently only supports execution of ELF Linux binaries, but given SimEng can also be built/run on MacOS it would be useful to also support Mach-O binary file types. This is also more relevant with the introduction of Apple Silicon CPUs which are based on the AArch64 ISA.

This will likely depend on Issue #324 given it is difficult to statically compile on MacOS.