GS/DX12: Compile shaders to Shader Model 6.5

[TheLastRar]

Description of Changes

Uses the DirectX shader compiler (DXC) to compile shaders.

Rationale behind Changes

Newer shader models could potentially allow new or improved features to be used with DX12.

The librashader (#14433) and (currently) the Uber shader PR (#14737) both already bundle the DirectX shader compiler.

Initially, only support Shader Model 6.5 (in addition to 5.1 with FXC), as, with the exception of QC 8cx 3, all SM6.0 capable devices support at least shader model 6.5.

While we could compile DXC on the CI, I've opted to extract from the nuget package for CI speed.

Suggested Testing Steps

Test the DX12 backend
Test CAS on various backends

Did you use AI to help find, test, or implement this issue or feature?

No

[mirh]

FWIW while I was researching UE5 dx12 compatibility (which afaik is also sm6.5+) I found out skylake graphics should be missing it too, because unlike the newer gen9 models its driver was left stuck to wddm 2.1.

[JordanTheToaster]

FWIW while I was researching UE5 dx12 compatibility (which afaik is also sm6.5+) I found out skylake graphics should be missing it too, because unlike the newer gen9 models its driver was left stuck to wddm 2.1.

Then it will use SM5.1 as you can see in the changes.