Improve Vulkan AMD GCN detection and build controls

[rasbid]

Summary

• allow forcing GLSL extension detection results when building the Vulkan backend and document the cache variables
• relax AMD architecture detection so Polaris-class boards are classified as AMD_GCN when shader core properties report a 64-lane wavefront
• document a Vulkan build recipe and runtime flags tuned for AMD GCN (GFX8/Polaris) GPUs

Testing

• not run (Vulkan SDK/glslc unavailable in container)

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https://chatgpt.com/codex/tasks/task_e_68d2849b914c8330a03464e6383fc866