GPU: Add support for SM7 SPIR-V

[flibitijibibo]

Because I know somebody's going to file this eventually:

https://devblogs.microsoft.com/directx/directx-adopting-spir-v/

There will probably be some catches in the same way that HLSL has some catches to support compiling to Vulkan-friendly SPIR-V, but long-term we should be able to add this to D3D12's supported shader formats when SM7 is supported. Hoping it doesn't require a new feature level, but who knows...