Fix D3D12 compute

[alexreinking]

We had stopped testing D3D12 because winbot 2, which no longer exists, didn't support it. See halide/build_bot#260

However, after enabling DirectX Compute Shader testing for #9077, it seems the backend has bit-rotted. This PR aims to fix the existing support to make validating that PR easier.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[alexreinking]

This is passing. The failure is due to the extra sm60 target in the buildbot config