ggml : add BF16 unary, GLU, binary and scale ops (CPU, CUDA) (#29675)

* ggml : add BF16 unary, GLU, binary and scale ops (CPU, CUDA)

* ggml-cpu : use per-op _bf16 functions for BF16 unary and GLU ops

Assisted-by: Claude Opus 5.5

* CUDA: use ggml_cuda_cast in binbcast and unary kernels to fix the HIP bf16 build

* ggml-openvino : reject BF16 SCALE and mixed-type BF16 ADD/MUL/SUB