ggml: add FLOOR unary op (CPU + SYCL)

[safranowith]

Summary

Add support for the FLOOR unary operator to ggml, with implementations for both CPU and SYCL backends.

Details

• Added GGML_UNARY_OP_FLOOR to ggml_unary_op enum and updated operator name table.
• Implemented CPU version in unary-ops.impl.h.
• Implemented SYCL kernel in element_wise.cpp and dispatch in ggml-sycl.cpp.
• Updated ggml_backend_sycl_device_supports_op to allow FLOOR.
• Extended unit tests to validate the new op.

Testing

• Built with -DGGML_SYCL=ON -DLLAMA_BUILD_TESTS=ON.
• Ran ctest --output-on-failure: all relevant unary tests passed on CPU and SYCL (f32).
• Verified correctness of FLOOR outputs vs. CPU reference implementation.

Notes

• f16 support on SYCL GPUs may depend on driver/device capabilities (currently not supported on some Intel GPUs).
• f32 works as expected across backends.

[safranowith]

// FLOOR: rounds each element down to the nearest integer.

[jeffbolznv]

I don't see the changes to test-backend-ops.

[safranowith]

Tests have been added to the test-backend-ops file.

[CISC]

#16613