[aot] Support Vulkan layers on MoltenVK platforms

[PENGUINLIONG]

Vulkan layers cannot be loaded by directly loading libMoltenVK.dylib because it's merely an implementation of Vulkan driver. This issue is a major obstable on the provision of print in the Vulkan backend. Link to the ICD loader libvulkan.dylib instead.

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[PENGUINLIONG]

cc @ailzhang You might wanna check if the Python wheel is still working on a machine with no Vulkan SDK nor MoltenVK installed.

[ailzhang]

Thanks! I've verified that this PR enables loading vulkan validation layer on macos, although printing still doesn't work as my M1 machine doesn't support non semantic info extension :(