venus: workaround a gcc-15 dead store elimination (DSE) bug

No issue with clang or gcc-14.x (or earlier versions). The issue only
shows up since gcc-15.1. The compiler somehow fails to consider those
cs helpers dereferencing the pointer from the pNext chain for reads,
and thus has falsely optimized away the pNext store. This change works
around this with a no-op memory clobber.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/13242
Cc: mesa-stable
(cherry picked from commit b0397b967d)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39969>

.pick_status.json

@@ -934,7 +934,7 @@
         "description": "venus: workaround a gcc-15 dead store elimination (DSE) bug",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/virtio/vulkan/vn_device.c

@@ -79,6 +79,10 @@ vn_queue_init(struct vn_device *dev,
       .queueFamilyIndex = queue_info->queueFamilyIndex,
       .queueIndex = queue_index,
    };
+#if DETECT_CC_GCC && (DETECT_CC_GCC_VERSION >= 1501)
+   /* Workaround GCC-15 aggressive Dead Store Elimination (DSE). */
+   __asm__ volatile("" : : "g"(device_queue_info.pNext) : "memory");
+#endif
 
    VkQueue queue_handle = vn_queue_to_handle(queue);
    vn_async_vkGetDeviceQueue2(dev->primary_ring, vn_device_to_handle(dev),