Cross-building for PPC or "Apple Silicon"

[René J.V. Bertin]

Hi,

This is a development question but not about MacPorts ports so I'm posting it here:

I'm currently working on another CMake contribution that aims to check whether the selected compiler(s) can and do target the architecture(s) selected via CMAKE_OSX_ARCHITECTURES. The idea is to extend an existing internal MachO header parser and use libmacho to translate the numeric CPU type to an architecture string.

My question is to what extent I would need to complete the architectures known my the system libmacho on older systems. I see that the MacPorts LegacySupport headers define CPU_SUBTYPE_ARM64E if not already defined but not why. I also noticed that my own linkers (system & MacPorts installed) on 10.9 don't allow me to build PPC binaries though I can still generate PPC (but not PPC64) object files using the 10.6 SDK.
Can I assume that libmacho knows the names of all architectures its OS version can hope to build for? Or is it trivial enough to set up, say, a not-so-recent Intel-based system to target ARM64 even if it cannot do that out-of-the-box? And by "trivial enough" I do mean to warrant that the necessary supports gets added to stock CMake.

FWIW, I had a look at building the libmacho sources included in the port:cctools tarball. I managed but they're clearly not (or even less) meant to be built & user-upgraded like the other binaries so I won't be teaching CMake's own build system to look for libmacho (the functions in the system version are accessible through libSystem, so without need to link libmacho explicitly).

Thanks,
R.