Add craft configuration options for Mac arm64 builds

This is the first of a few MRs I have planned to support Craft builds for MacOS arm64 (M1).

I currently have an arm64 KDE Connect building under Craft, so this is possible 🥳

This one is pretty simple; I thought it would be good to get the naming of the abi & craft compiler methods agreed upon (as well as the general idea of supporting arm64 mac builds) before I tidy up the rest of my changes.

Cheers!

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Future steps:

• Add a warning that compiling arm64 is experimental?
• Use the new method for detecting a arm64 build to add relevant compiler flags to meson, cmake and qmake (e.g. -DCMAKE_OSX_ARCHITECTURES=arm64)
• Add patches to blueprints that don't yet have M1 support (pixman, which has an upstreamed commit to do this and libpng, which does not)
• And various other blueprint problems (can't use SSE2 in libfftw, openssl and qtbase need some fixes to manually compile themselves)