cuda : fix build on cuda 12.8 (#18672)

compute121 requires 12.9

Signed-off-by: Oliver Walsh <owalsh@redhat.com>

ggml/src/ggml-cuda/CMakeLists.txt

@@ -47,7 +47,10 @@ if (CUDAToolkit_FOUND)
                 #     check Modules/Internal/CMakeCUDAArchitecturesValidate.cmake in the CMake git repository instead.
                 # However, the architectures 120a-real and 121a-real should work with basically any CMake version and
                 #     until the release of e.g. Rubin there is no benefit to shipping virtual architectures for Blackwell.
-                list(APPEND CMAKE_CUDA_ARCHITECTURES 120a-real 121a-real)
+                list(APPEND CMAKE_CUDA_ARCHITECTURES 120a-real)
+            endif()
+            if (CUDAToolkit_VERSION VERSION_GREATER_EQUAL "12.9")
+                list(APPEND CMAKE_CUDA_ARCHITECTURES 121a-real)
             endif()
         endif()
     endif()