No debug info on Darwin

[int3]

macOS / Darwin builds store debug info separate from the main binary, in a .dSYM folder. However our caching system does not support more than one build output.

I think we should fix this by expanding the caching system, but that can probably wait until we upstream the rest of the CPU code.

[minjang]

What do you think contributing this feature in the upstream first?

[int3]

I mean it's largely a CPU-specific problem since the caching only affects CPU kernels. But as my task description was saying, I think we can wait until we upstream the entire CPU backend first before working on this, I think it's pretty low-pri. Just filing to keep track of things.