The usage of __builtin_unreachable after calls to quick_exit were distressing.
If a function is properly marked [[noreturn]] then __builtin_unreachable is not
necessary.
Looking into this further, we seem to have header only implementations for CPU
targets. The inline nature of these functions is curious; we're going to exit,
it doesn't matter if we need to pay the call of a function or not. If we just
make these functions have distinct TUs rather than be header only, we can clean
up the cmake rules for quick_exit which were different between CPU and GPU.
Remove darwin support for quick_exit. This isn't being tested, and we can bring
it back when necessary.
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