Support for universal binaries containing Metal code

[maleadt]

Obviously depends on #7, but in addition I'd like to be able to parse FAT binaries that contain GPU code. Example of such a binary attached (zipped because of GitHub restrictions): test.zip

❯ file test.bin
test.bin:  [applegpu_g13s:Mach-O 64-bit GPU executable applegpu_g13s] [air64_v25:MetalLib executable (MacOS), version 1.2.7]
test.bin (for architecture applegpu_g13s):	Mach-O 64-bit GPU executable applegpu_g13s
test.bin (for architecture air64_v25):	MetalLib executable (MacOS), version 1.2.7

An example parser of such binaries can be found at https://github.com/dougallj/applegpu/blob/98008ca1c6f413706e57465f8a4116021501196f/compiler_explorer_tools/metal-archive-extractor.cpp

Naively adding the new magic numbers and using #7 doesn't just work, so I'll have to take a closer look at the parser 🙂